There are many reasons to blog.

1) Money- Some people are paid to blog, and blogging creates income with advertisements and affiliate programs

2) Adding to your portfolio- Blogs are an easy way to add to your writing portfolio

3) Promotion- I use my blog sometimes, to promote my articles on other sites to increase their page views

4) Networking and making friends - Blogs are an easy way to network with other writers. It can also be a social way to make friends.

5) Stir up your creative juices- a blog can be used to stir up your creative juices so you can write an article