not an expert, but i read that stretching is actually what it sounds like--stretching. the benefits of stretching being to elongate particular muscle groups, which adds flexibility. this is something like exercise, it takes time to create flexibilty. i read flexibility can increase muscle power in sports in which a greater range of motion delivers more power. not sure that applies in cycling, since you are limited to a specific range of motion. but i'm waiting for replies of those disagreeing, i like learning.

warming up is also very specific, you are increasing blood flow to the muscle groups (including heart) that you will be using more intensely during the ride (or whatever activity you are doing.) i just start out more easily on my ride.