This is not a definitive answer (a quick googling failed to turn up a solid source), but presumably they have to ignite both the gas generator (that powers the

This is not a definitive answer (a quick googling failed to turn up a solid source), but presumably they have to ignite both the gas generator (that powers the turbopump) as well as the main combustion chamber. You certainly see the green flash of TEA/TEB lighting underneath the rocket (inside the combustion chambers) just before liftoff.