This is very important information for both beginners and experienced meditators.
On the cushion
For beginners, the most important fact to remember is that meditation is a training, a cultivation. It is a training in dropping thoughts, in letting thoughts go (instead of ‘automatically’ falling into long chains of mental proliferation, one thought leading to another with us convinced that the next thought was the only thought that could arise).
As with any training, specific setups might make learning easier, hence the specific instructions regarding posture, vigilance, suggested object of focus, etc. This is similar to learning an instrument: you don't start as a soloist with a whole orchestra in your back preying on your mistakes! It doesn't mean you won't become a soloist, but it's not the setup in which you learn at first.
So it is important to learn meditation before you feel the need for its
benefits. It is when your life is (relatively) more peaceful than it's a
good time to cultivate even more peace. It is not when your life is a chaotic
conflict and turmoil that you have the appropriate setup to learn!
This is all relative of course: even the tiniest bit of peace can be cultivated. What matters is that you understand that cultivation is to happen whenever it feels things are getting better. However, your mind will tell you the opposite! Your mind will tell you: “things are getting better, why would I need to meditate now? I'll meditate when it will support me, i.e. when things will worsen.” It doesn't work this way.
Let's assume you learn to play an instrument: circumstances put you on the spot and it is announced you'll be the soloist in an upcoming concert, you know you're not ready and pressure builds up. Then a more experienced player turns up, and it is announced that she will be the soloist and you'll join the rest of the orchestra. Pressure subsides; this is more within your capabilities. At this point, is it the time to say “things are getting better, I won't massacre the whole concert, so I don't need to practise my instruments until I'm asked again to be a soloist”? Of course, not! Enjoy that you're no longer on a disastrous course of events, but realise this is an opportunity to practise more, not less!
Outside the cushion
For advanced practitioners, once you're able to meditate “outside the cushion,” the key to consider is ‘appropriateness’.
For example, is śamatha (‘calm-abiding’ or ‘concentration’) a form of meditation you should do while driving? Absolute not! It is not appropriate! When you drive, you are to be open to the world and notice what arises and requires a wholesome response. You cannot focus on the car in front of you, and ‘miss’ the pedestrian who didn't pay attention before crossing the road. You cannot focus on your breath, and ‘miss’ a red light simply because there was no physical obstacle forcing you to stop. You cannot become so engrossed in listening a speech from a teacher that the rest ‘disappears’; as a matter of fact, the only sounds you should pay attention to are from the environment, you shouldn't try to cover them by playing musics, speeches or news, or by having hand-free phone conversations (these are all distractions, explicitly!).
While driving, śamatha is a classic case in inappropriate time and place,
inappropriate here&now; this is not a manifestation of wisdom.
Now, most likely, some streams of consciousness in you will know so and will force you to pay a minimum amount of attention to the traffic, while other streams will try to bring this ‘distraction’ to the object of meditation! This is one of the best recipes you can have to generate headaches: you created a conflict within your own mind, by choosing a form of meditation inappropriate for the context. The fact that you can, doesn't make it appropriate.
What's appropriate while driving is “being present”. Hence, driving —like doing the dishes, looking, chopping wood— is meditation! However, the object of attention is the whole world moving around, without focusing on anything particular object (susceptible to make you miss the arising of another phenomenon)… hence driving is a form of vipaśyanā meditation, not śamatha.
Driving is a situation when you want to be open to the world, want to react appropriately to the situation at hand, regardless of what the situation was earlier or of what led to the current situation (it's not because the traffic flowed fast until now that you shouldn't slow down), and regardless of where you go (no, it's not because you're late for an appointment that you should drive faster; no, it's not because you want to turn right here that you may ignore the pedestrian in the way; no, it's not because others are ignorant that you should respond to their honking by appropriating their delusions that the traffic ‘should’ move faster). This is vipaśyanā, insights into the three marks of existence, into the impermanence and unsatisfactoriness of every conditioned thing that exists and into non-self: yes the traffic is unsatisfactory, no you're not the centre of the universe, yes what was safe a moment ago may no longer be safe!
Yes, by being mindful, you can notice whether your breath is accelerating because you're stressing yourself by driving too close to the car in front of you, or you can notice both the other drivers and the narratives you make about them (“he's such a bad driver!")… But being mindful is not being single-pointedly concentrated. You can be aware of narratives and drop them while still paying attention to reality as it is (he may not be an intrinsically-bad driver, but his trajectory right now is very dangerous all the same). The anchoring, though, is just an anchor.
For buddhists, it is clearly said that śamatha does not lead to Wisdom;
it is an extremely useful tool, a key tool worthy
of a specific mention in the presentation of “eightfold
path”, but it is a tool to learn not
to cling, not to follow mental proliferations.
Once you learnt how to drop thoughts quickly after they arose, or as soon as they arose, once you learnt not to feed the thoughts (hence let them naturally subside, instead of perpetuating them by appropriating them as ‘yours’), śamatha has provided his educational value.
You can still practise śamatha, just like professional soloists continue
to practise scales, but you cannot confuse scales and inspired music that is
an appropriate response to the context! A concert during which the soloist
would play scales is unlikely to be a huge success (unless it's some
experimental ultra-modern music?).
You can still practise śamatha, but you have to practise it appropriately. The rest of the time, you can embody the lessons (you can e.g. drop thoughts and avoid mental proliferations while driving); it's not the same though as dropping the whole context, not the same as the exercise to learn to drop thoughts.
The main risk for advanced practitioners is to see śamatha as a tool for relaxation: “driving is stressful? Let's single-pointedly concentrate!" Relaxation is not an appropriate use of a tool of which the purpose is to learn how to drop thoughts, to learn to feel when clinging arises, to learn to feel when thoughts proliferate and lose contact with reality. Relaxation is a side-effect, and it doesn't work to use śamatha as a tool for relaxation independently of the context, and the headaches that will arise should give you a clue that you're not ‘relating’ appropriately to the world: nirvāṇa is sangsāra; nirvāṇa is not a different place, it is a different relation to the world, a relation born from understanding and realising causality, the universal wish of sentient beings to be free from suffering, and appropriateness / wholesomeness.