why doesn't my friend just send me the pictures directly?

Um . . . because if you post the pictures somewhere then you can share them with all of your friends at once without jamming their email with large files. Seriously, I'd much rather my friends just sent me a link than that they emailed me pictures all the time.

It depends on what you want out of it, really. You can link to music, etc. on MySpace (and probably a lot of the other sites), and not everyone has the same programs to play music files. Links are easier. Blogs, etc. are great for people who want to share ideas and photos with a group of people with similar interests; you can share them with everybody and have them in a coherent order rather than an email with a bunch of pictures appended to the bottom.

If all you want is email, fine, but there are a lot of other people who have good uses for blogs, etc.