Redownload games any time you like (as many times as you like), in game overlay (pres **** +tab, and then you can browse the internet/talk to friends in it, can also play flash games so you can play games while you play games), no need to reinstall after a format you can just copy paste the whole steam folder.
Lose your disks and have limited internet, you can just copy the folder from a mates pc, all games auto update and if you like that sort of thing there's often achievements.
Also any game that has "steamworks" means you have to use steam anyway even if you bought retail.