It's why we use a bit of jargon in user experience (UX).
We call "why users do what they do, when, where, how, and what they do it with" context of use.
It's a crucial piece, or, rather, set, of information.
If you know who your users are, what they're trying to get done, why the want to do it, and how they will interact with it – including the environment and circumstances they're in – you will have a really clear picture of what your product or service needs to do to make that happen.
It's critical to know, because – well, basically, if you don't know this stuff, failure is an absolute certainty.