If I remember right, Race Driver: GRID (long since delisted) has no mouse support -- not even in menus. (Not that mouse would be any better for steering or acceleration than a gamepad.) Try playing something like that with pure keyboard controls. (I did. It wasn't fun.)
I'd also say that any sort of game where a major part of the gameplay involves being able to aim independently from your character's movement/facing direction, AND where finer movement control (which keyboard-based movement can't handle well) is required, is going to play better on a gamepad. (For example, WASD movement in an angled/"isometric"-view game like Bastion can be awkward...particularly if there are deadly hazards to be avoided in narrow areas where you're also being attacked.) That's not to say you can't play it well with keyboard and/or mouse; it's just that, for someone who's got experience with both types of control, gamepad will be better. Just like mouse and/or keyboard is going to be better for lots of other types of games.

Of course, this is leaving aside cases where mouse and keyboard controls (or, more rarely, gamepad controls) in a particular game are suboptimal not because the type of game made that inevitable, but because the devs had a notion of one "correct" input method, and didn't bother doing much work on the other one, then just told players to use their [i.e., the devs'] preferred one. (Broforce, with its unnecessarily crappy keyboard controls, springs to mind here.)