Steam Benefits
* when you buy a game on steam, you can download it onto however many computers you own. Even better, the games are stored inside your steam folder. You can copy this folder to another computer, click the steam.exe, and withing a few minutes, have all of your games accessible. No need to dig up dozens of disks, spend hours redownloading, etc.
* some steam games support steamworks, these store your savegames in the cloud, so you can log off your desktop, jump on your laptop, and resumke the game from the same point.
* Games at release are often a little more expensive than at retail. However, steam has a one game on sale every day, an extra game at midweek, one or more extra deals at the weekend, and several times a year they have massive, massive sales, where most of the catalog (hundreds, maybe thousands of games) are reduced, by 66%, 75%, even 90% or more in rare cases. Steam sale events are awesome (not so good for your bank balance).
Here's an example of a
weekend event
Then there are the achievements, game forums where you can check out people's opionons of games before you buy them (and so avoid stinkers), the in-game overlay which allows you to SHiFT+TAB out of a game to chat or visit a webpage, and much more.