As for me, it is mostly because I am interested in feedback from outside the bubble of people I work with every day. (I have to admit that, if the topic is appropriate, I still prefer Stack Overflow and some other Stack Exchange communities because I can reach more people there. On the other hand, most interesting questions are off-topic there.)
How about you? Especially those of you who put lots of work into uploading elaborate tutorials and essays.
I want to help people learn. I think the best way to solidify understanding is to be able to explain it to others and I think it's important to give people approachable and accessible ways to learn concepts. The more ways we have to learn the same concept the more likely it is that any given person will find a teaching tool that resonates with them.