It's a driving sim that mainly focusses on racing which also means that you can simply drive cars around a track for the sake of enjoying driving cars around a track (which I often do if I'm not racing online).
GT always included humdrum cars from the start, that's actually one of the main things what set it apart from all other racing games before that.
Also, I didn't necessarily imply taking low-powered cars for a Sunday drive but the fun that can be had racing them against other low-powered cars, it's often far more tense than racing high speed machinery against each other.
And another thing, there's also fun to be had by other factors like handling, sound or visual aspects (the combination that is) which doesn't automatically means it needs more than 150bhp to be enjoyed, at least not by me.
Just to be clear, I like a lot of different cars for a lot of different reasons, I don't have one set of criteria which all cars included should have to follow, I want a lot of different experiences within one single game, not a huge selection basically offering the same thing with slight differences.