There are a bunch of different types of blogs and, from what I understand, certain blogging sites cater to certain types of blogs although not exclusively.
There are your journal/diary-type blogs that I've found many younger people write and meander on about their day. For those, it's dependent on the person why they keep coming back to read more. Personally I don't want to read about someone's day to day ramblings especially if they're not interesting. In fact, the only journal blog that I read is melaniehoo's but specifically because it's interesting and she has a good voice throughout. It's also not a "normal" blog either in that she's from the US and emigrated to Mexico with her husband and it's about that. Very interesting and quirky.
There are blogs with specific themes that they stick to, others with just tones that they keep throughout. Mine, for instance, if somewhat observational only because I don't know what else to call it. It's humor and I'd like to say my voice and the content I write keeps people coming back and I have it to expose people to my voice and give update on my writing.
Some blogs I just don't get and don't understand why people start them. I think, outside of a bonafide purpose like writing, people do it in order to gain internet popularity, to feel wanted and loved, I guess. It's an ego-boost to have people read what you're writing and encouraging you to write more and some people need that.
There are so many different reasons to have a blog and I didn't have one until JeanneTC urged her fellow humor writers to get one as a means of promotion. I didn't get the point at first because I always equating blogging to emo kids complaining about eyeliner but now I see it as a means to hooking readers before there's anything to read.