Like Track_Veteran says, they are bits of information about automotive history, with related imagery. Depending on each person's interests, some museum cards will be boring and others fascinating.
Frankly, I bought GT5 to play a great driving game and museum cards are mostly a waste of space, save for a few truly pivotal and noteworthy events. If I wanted to learn automotive history, I would have bought the internet.
If GT is a driving game, why are there no actual roads? Sure, there are street tracks like London and Tokyo, but a "driving" game would have traffic, stop signs, etc. The GT world is made up of 99% race tracks, therefore it's a racing game (simulator).
To race: a contest of speed. GT is made up of races, therefore it is a racing game. Yes, to race means you must drive; but if we only "drove" that means we would be missing the point of practically the entire game.