When your company starts growing, it's harder to keep everyone up-to-date with everything that's going on.
Your monday meeting that includes everyone can only work for so long.
It becomes more difficult for everyone to have a deep understanding of all that's happening. It's even difficult for the CEO at times.
You need to be really thorough with your company's plans: the broader company strategy, the product strategy, the marketing strategy, or whatever.
It's important to lay things out in a clear and concise way so that everyone understands what's happening.
While you could get away with not writing down a lot things when you were small, you need a "source of truth" for everyone to be able to refer to as you grow.