Well, there is so many reason to answer that questions.
~ to shared ~
Some people liked to share their information. This blogger so helpful to reader. This kind of blogger usually share their information based on their experience by them-self or maybe from people who close with them.
~ to get money ~
Well, this is one of the most reason people finally become a blogger. To get money. By what? Ads!
yep, some company choose a blogger to advertise their product because usually article in the blog is honest. Blogger wasn't tell a lie when they truly experience the things. So as a blogger you should to be honest! Especially to yourself *wink
~ to be famous ~
The previous blogger is famous nowadays and finally you decided to make a blog so you can be a famous too?it is a big deal to become famous because a blogger never ever think will become famous one day. But if you wants too, you must working hard by looking what people like, try to study consumer behavior maybe it is necessary. :)