On the road you get breaks. Coasting, lights, downhills, curves, easing up. In the spin class you never get a break. You pedal as hard and as long as you want or can, without road conditions to worry about. Also, on the spin bike you can crank up the resistance to any level. Sometimes I will turn it up until it is very hard to turn the pedals over, it becomes like lifting weights, I'll do this for a while each class. It has definitely helped my leg strength. To duplicate that on the road you'd have to find a meaningful hill and go up in 53 x 12 or something equally daft.