There are multiple reasons to set up a blog. Working online is probably the main reason people start a blog. Working online generally consists of Affiliate Marketing, but can be used to give advice on couponing/hobbies filled with Affliate Links that make the Blog Owner money.
Others use blogs as a personal diary or as a way to keep family and friends updated. These are generally kept private.
If you are using your Blog to make money, keep it informative and fresh. Search engines will pick up on good information, which in turn will eventually help your search engine rankings. The best thing to do is post at least daily.
Do not spam your link, nor post illegal things like pornography. Doing those things are the best way of having your site banned and ip banned.