An online diary is the short answer. As someone who reads a number of blogs and maintains one, I can say that common reasons include:
1.) Diary of daily life - Especially common with people who have a unique job/life/hobbies (a farmer, wood worker, cooking, avid book reader) or people who have young children (sharing stories with relatives)
2.) Discussion on random subjects - Professors and enthusiasts sometimes maintain blogs related to subjects in computers, biology, math, science, literature, hobbies
3.) News - People both agregate news on a certain subject or offer commentary (Pharyngula, mentioned about, is mostly of this type)
4.) Marketing - Companies maintain blogs of new products, contests, new services, etc.
5.) Entertainment - Humerous stories, comic strips (webcomics), etc.
Blogs, especially those related to unique hobbies and interests, tend to form communities which comment on each other's blogs, offering ideas and encouragement. It's a form of social media I suppose, but more content-heavy and less frivolous than sources like facebook and twitter.
Finally, nobody has mentioned vlogs (video logs). On YouTube, you can find people who create regular videos of any of the 5 types above. These videos are usually organized in channels, and viewers can offer comments or create their own videos as direct responses. Just like extremely popular blogs can earn you money (through advertising and referrals to online purchasing), vlogs pay money through Google text ads. One source recently estimated the income of the top 10 "video bloggers" and many of them landed in the $100,000+ range.