For the most part I'm the opposite- I usually use KB+M even in games that are probably objectively worse without a controller; Assassin's Creed for example. I would never try to argue that Assassin's Creed isn't better with a controller...but I play with KB+M anyway. I just...do.
EDIT: Thinking about it some more...I definitely use a controller for character action games like DmC or Bayonetta.
But the most common thing I'll plug in a controller for is first person games that aren't combat-oriented (Stanley Parable, Gone Home, The Witness, etc). What I love about those games is the sense of place and the feeling of occupying and exploring a space, examining small details. That atmosphere is hampered for me by the binary movement speed of a keyboard (you're either standing still, or you're pressing W and moving at full walk speed)* and the quick, darting movement of mouselook.
It seems like a really silly little thing- but in those games I want to be able to walk slowly, I want to be able to pan my view around gently.
*Semi-related: The PC versions of the early Splinter Cell games mapped multiple steps of walk speed to the mousewheel. Movement speed was important for stealth in those games so I always thought that was a smart way of making sure PC players still had the ability to sneaky movement. Haven't seen it in any other games, to this day.