Because you can see so much of someone's personality before you start to chat with them (assuming they tell the truth in their profile) and they can see some of yours too. It means that you avoid trying to tailor yourself to meet what you THINK they want because you've already committed yourself to certain likes/dislikes etc.
Anyway it worked for me I met the most wonderful woman despite having profiles like chalk and cheese!
Because they are less personal so people are more likely to try a little harder when it doesn't mean as much to be turned down. It is also easier to find people on the Internet that are looking for the same things
All those things emails,pic's,phone are used to get to know someone then you can get to know them on a personal level in person after all the other stuff.