Why are there multiple version of Magic The Gathering Game on Steam?
You appear to be talking about Duel of the Planeswalkers, a game based on M:TG where you start with a number of decks and you unlock cards to add to those as you progress in the game. There were different versions for the past few years. Each version had new decks and features. You can compare it to several sport games (NBA 2k15, WWE 2k15), they all get newer and sometimes better versions each year.

You can, however, keep using the older versions. But newer versions contain newer (and sometimes more) cards, new enemies, new battle types etc.

There is also Magic: Online, a true digital equivalent of Magic: The Gathering. This is a version which stays up to date with newer cards, but can be more expensive than the DotP versions you can buy (eg) in Steam. In MTGO, you create your own collection and play against other players, not against AI opponents.