Because as you mentioned you can handily beat the AI with a comparable car, so, many of us enjoy taking a lesser car in to make things interesting... think 200 point races from GT4.
When you are in such a race the qualifying is important no to ensure a pole position, rather to tune the car for the track.
My racing buddy and I used to take a 305hp ammy rx7 into the 450+hp jgtc races... use qualifying to dial the car in (without ever finishing the 2nd lap, aka, no chance to actually qualify) and then race.
It was essential to have each gear set to match the corners of each, and dial the final drive in as well to each track, and without being able to qualify (actually, practice) it's a guessing game.