Difference between 650c and 700c wheels? I've seen triathalon setups with either 650c or 700c wheels. What are the pros and cons behind both wheel sizes?
They are primarily used for high-performance road bikes for smaller riders. There comes a point where a proper fitting frame for a smaller rider will have strange handling using 700c wheels. 650c wheels are designed to address that.

Smaller riders can also have road bikes built with 26" wheels.

So if you are of average height there is no significant advantage to 650c wheels.