yes but you do not need to download the game, you just have to install it locally with the disc. The op said he didn't download it, thus he probably bought it from a shop (or pirated it)
Steam is required to decrypt the games. Games on the discs that Steam sells are always encrypted and the decryption keys are released on the release day to prevent early sales.
Even if you consider installing from DVD not 'downloading' from Steam. Steam automatically updates games for you and there has been an update on Portal 2 since release.
Jamstruth said:
QUOTE(DeMoN @ Apr 23 2011, 09:24 PM) If he pirated it then I'd hope he wouldn't have the idiocy to ask why online play doesn't work.
Uhm... There are 2 things I believe infinite. The Universe and human stupidity, but I'm not to sure about the Universe. Einstein sums it up.