it is installed when you install, Counter Strike. It is the software the company bundles. It has news updates etc. You can also download demos from it.
I don't mind it too much but it's stupid because say you don't have the HL2 disk or the CS:S disk, or something of the sort, because you credit carded it off www.steampowered.com
Now you have to DOWNLOAD all that?? That's a good 5 giabytes or so of stuff. Good luck waiting all day. I'm on a 1mbps LAN because the connection is split with 6 computers and a wireless connection, and just one update for the game, like those skins can add up to 10 minutes. Now imagine downloading the WHOLE GAME. It literally wouldn't be worth it. Why now create the site so that it uses the Valve servers as download sites so that you can just download a .zip and cut down like 75% of the file size down. No, no, no. They have to create an annoying, resource eating thing. Valve is stupid.