What is the use of GPS/Elevation on Bike computers
Ever had that "this has to.... be the top.... oh sh$%.... it's a crest.... there's more...." feeling. Knowing altitude lets you pace your climb and arrive the the top without over (or under) doing it. 

Imagine riding flat roads without knowing speed or distance - it can be done, but to maximize training effort or race performance you need to know speed/distance on the flat and altitude/climb rate on the hills.