If you do not own a blog right now and want to know why many people are blogging. The main reason is to earn money. It is very practical for us to look for ways to earn money and a very easy way is blogging. Many successful bloggers nowadays are earning more than 20k USD monthly already. This is why blogging can be considered a business and a job. Your boss and employee is yourself.
Another reason is for fame but this is still second in the line to earning money in blogging. If you have a very informative blog reader will eventually look for the author and know more about him/her, this is for them to ask something or suggest ideas to the blogger.
And the third reason is for fun, some blogger just blog for personal amusement and a past time. But blogging can also be the best past time just because while you are enjoying you are earning money and making a name in the blogging and internet world.