1.You can host servers(Unless your internet is cable or DSL wich need to be portforward'ed)
2.Meeting new friends
3.A wrench system that can do almost everything
4.Play with your friends online
5.Meet new friends, join clans and maybe make one yourself
6.Build alone or with your friends something big
7.Join over 100 diffrent servers
8.Deathmatch builds can be realy fun
9.Share your creations Via blockland forums and post them on Gallary for people to comment
10.Like Otis said, you can build up to 128,000 bricks hoping that your computer isn't 20 years old