Changing gears with a keyboard is nearly impossible, with a pad it can be done with practice, problem with the pad you're using your hands, well fingers... You need to get a techinque that works, and some just can't press the accelator and change up, then brake and change down, then corner and hits the apex's right, or even pull off an overtake all at the same time with a pad..
Steering wheel means you use your feet to speed up or slow down, and use the wheel to turn, and your fingers to press the flappy paddles if you're wheel has them, its actually easier with a wheel.. Its more natural..
I'm maybe going to have to invest in a wheel stand. I'm a fairly decent racer, but I hate changing gears with a pad, you always end up downshifing or upshifting too early or too late.. and in a F1 game, every mistake can cost vital seconds.. not bad when racing as a rookie, but crank up the AI, and it becomes really tricky. With a wheel you have more of a chance..
upon saying that, some can really race, and hold their own with a pad, but most top lappers use wheels.