Some do it for recognition or acknowledgement of their peers. Others because they think it will help their readers - that they have something useful to say. A few even do it for money, though it is often easier to earn a better salary in more straightforward jobs. Then there are those who write 'because they have to'.
This last group is the most interesting to me. Writing because you have to, seems to be the purest form of expression - there's no need to be rewarded, patted on the back or given money or fame or whatever. It's the process that matters, in a kind of Zen, do it and enjoy it, way.
This goes against everything we teach in Creative Writing or art classes generally. It's always pushed as a trade or profession - look at the great writers (or artists or photographers) and what they have done. But the reality is that most writers earn very little, well below the average UK salary and less than the minimum wage if you take all the hours into account. Like footballers and actors, the occasional multi-million earning novelist implies that is what all writers make, whereas most earn less than £13, 000 a year.
That suggests it is the love of putting words down that must drive many who stick with it. It's much easier to earn a living as an accountant or office manager or IT advisor. Many in writing are educated enough to do either of these jobs.
So writing is a vocation and not just a job. Write if you have to and say what you believe in. And if you don't have to, try accountancy or IT advice or management. The money is much better.