Ultimately, you should write because you want to. Sometimes, though, you write because you have to. It often coincides that you want to and have to write.
Some people find writing therapeutic.
Others find it a way to spread their thoughts across the world, and thereby conquer all. I don't know. But you should come to undergrowth to find out. If you've read all or some of these pages, and undergrowth appeals to you, you should come to a meeting. If you like coming to undergrowth meetings, you should join.