I'm a seasoned Kerbal Space Program player, so I'm not asking about why you'd want a stack-launched vehicle to seek a specific ascent-orientation for reasons of dV or maximizing launch-abort options (though I anticipate that others reading might benefit from a synopsis of those relevant details.) I'm asking the question before that: why conduct a roll program while airborne at all, when you could simply a LOT of the design issues and abort contingencies by simply spinning the vehicle on the launchpad to simply be pre-oriented?

In other words, for a Space Shuttle launch, why not just launch the stack with the rudder pre-oriented on the desired easterly track? Why engage in the potential-for-screw-up ordeal of rolling while strapped to a giant hydro-lox firecracker?