The roll is made for two main reasons - as stated earlier it unloads the orbiter somewhat and prevents a structural failure and two, it allows an optimised trajectory.
If you send a launch vehicle straight up, you suffer horrendous gravity penalties but clear the thick portion of the atmosphere quickly, thereby minimising losses due to air resistance. Of course if you launch with a large horizontal trajectory the opposite is true. Therefore, the best solution is to head vertically initially and then start to introduce a horizontal component to decrease gravity losses and improve overall efficiency of the launch platform