Just to add my 10 cents worth, the shuttle roll programme has a few good reasons, and have been covered mostly above.
The main reasons that im aware of are:
Allow the vehicle to achieve negative q alpha -
this removes a large proportion of the stress loading on the wing areas, the weakest link in the complicated structural chain
This also helps the orbiter use the earths gravitational pull to accelerate, the big gain being that it gets faster quicker which allows a heavier payload to be carried into a higher orbit.
It increases the S-band look angles - the data antennae gets a better line of sight back to earth during the entire launch, seperation and entry to orbit phase.
If im wrong I appologise, this is from memory, but im reasonabily sure that I havent completely lost it!
Regards all.
Speedy
