Tim - at busy airports, start is 'controlled' by ATC who have the 'big picture' on aircraft movements. As keygrip has said, they may know of significant delays due to high departure or arrival rates and will try to sequence ground movements accordingly, so engines are not running too long. It also acts as a 'check' for the a/c AND ATC that all is in place - ie departure slot, ATC clearance received or on its way from the centre. At places where 'push-back' is used, they will also sequence this to fit with departure flow and taxyway bottlenecks etc. I expect a controller will answer with other esoteric procedures that have to happen (like fitting in cups of tea
)
ATC clearance depends on how 'big' the airport is and whether there is a separate delivery frequency/controller. You are right, it is not conducive to safety to receive the clearance while taxying, especially with the multitude of checks, door procedures, cabin preparation and company call to get final load figures etc etc, but it is a fact of life. It becomes even more interesting when operating single pilot!