According to Wikipedia:
blog is a type of website where entries are made (such as in a journal or diary), displayed in a reverse chronological order.
Modern blog are evolved from online diary and it often provides commentary or news and information on a particular subject, such as food, politics, or local news.
Ok, back to today topic, why blogging?
It's complicated. But I simplify it into two categories.
First category is to share the happiness, ideas, talents and secrets.
For example, you have a very nice joke, what will you do. Tell yourself? Or tell the whole world about this funny joke? Or you have a very good idea but none of your friend understand you genius idea.
Blogging is one of the best ways to share these information. The more people share with you, the happier you are.
The second group of people do blogging is to share their sadness, pressures, sorrows. Complain about the government, how stupid your boss is, or how stupid you are. Telling to friend may have some risk because they will blog it. They will get happier but not you.
Thus, the best way is to just put a nick name and express all you dissatisfactions to the world. The more people share your burden, the more lightly you are.
No matter which type of blog you are having, just blog it and you may cheer lot of people out there.