You could find a lot of articles, with 5,10 ore 100 reasons to start a blog but I think you can't write general reasons. Everybody should find their own, personal, unique reasons to start a blog.
You could start one in order to post recipes, or communicate with your friends and family, or you want to write about or work.
The single commune reasons is: you have to write something, to share a idea.
And I think that is also a useful exercise that will help you, in time, to express yourself better.