a stupid question about Steam

What’s the point of Steam, really? Suppose I were interested in picking up Portal 2. I have a choice, I can buy the retail version from Target or I could use Steam (which came pre-installed on DENT). Is the game full-featured in either case? Is there extra DRM in one and not the other? If I buy the retail disc, I can still install that on more than one PC, right? (I’m not going to game on anything other than PREFECT or DENT anyway).