Do Steam codes need a physical disc?

Under most consumer protection law codes around the world, there is the concept of "Fitness for a purpose". You would have bought the game disk expecting it to be fit for the purpose of being installed and run on your machine. If the game requires a Steam code for it to be run successfully, then it should have been supplied.

Since the Steam code was not supplied, the item you purchased is not fit for its expected purpose.

Since it is the Steam code that allows you to install (and download) a working copy of the game, this transaction could be considered fraudulent - obtaining financial advantage by deception - as the media you purchased does not include the license to use it, and indeed the seller could legally make and sell as many copies a he wanted (for the cost of the media plus postage) without violating copyright, as long as it was made clear that the transaction is for the media and does not include the license to use the software on it. Not mentioning this latter fact makes this transaction fraudulent.

A Steam code is all you need to activate, download and play a game on Steam (with some titles requiring activation with 3rd party such as Uplay for Ubisoft's games, but activation details for those are provided by Steam after you enter the code for Steam).

A physical disc is only required if you don't want to download the game or want to keep a physical copy of your game.

A steam code is a legal copy of the game, you don't need the physical disk. Also a boxed game with a used key (registered to an account) is pretty much useless, unless you just want it to sit on your shelf.