It depends on the target orbit. For launches to the ISS, you have to launch at the correct instant because even a few minutes later means the ISS is so far ahead it will take much longer, and use more propellant, to rendezvous.

I think they could probably delay a few seconds without too much trouble, but I guess the extra complication of recalculating the rendezvous isn't worth it because most of the time a few seconds won't matter anyway.