you purchased the game on DVD, that isn't making any sense to me.
It doesn't matter... the game uses Steam as its DRM, and also takes advantage of other Steam features such as an easy patch distribution system ans Steam Cloud.
As for the OPs concern about Valve going out of business... trust me when I say that not an issue any time soon. You're steam game list is perfectly safe. Not to mention Valve has already stated countless times that should they go out of business they would certainly make sure people could play all the games they have bought.
Though, it should be noted that when you buy a game you are NOT buying the game... rather you are buying the right to play the game (actually read the EULA if you don't believe me). So if the developer provides the game through a system that goes out of business and you can't play the game anymore... you're SOL and theres nothing you can do about it. But again, Vale (the owners of Steam) won't leave you hanging... they are the only company in the gaming industry that actually cares about its fan/user base.