There a couple of reasons for me.
First, I've always had a problem with self-esteem. It was particularly bad after I graduated from college where I felt like I was ready to give up on finding any real talent and thus finding anything of self-worth. I felt like I wasn't good at anything, so I felt ashamed of my existence. Writing is my creation of thoughts that no one else thought me how to express but me. When I've shared my writing with readers, they often say that I share my thoughts with great passion and inspiration. Finally, I finally found something I'm good at doing.
That leads me to my second reason. I want to use creative thinking and writing to express my thoughts to the world. I'm hoping in some way my words will help someone in some way. Perhaps there's a lesson I can teach them about living a good life or just helping a reader know about something they didn't know about before. In other words, my writing has helped me feel a lot better about myself over the years, so I'm hoping in some way it'll do the same for someone else.