As explained in the FAQs here, lights go on just as soon as there are any signs of germination. Can you put the trays under the lights sooner? Sure, it won't hurt but it isn't necessary.

Dave

I don't know what you are growing, but there are several varieties of flowers whose seeds benefit from or even require light for germination. Statice, snapdragons, scabiosa, and dianthus are examples. You'll also get a little bit of warmth from the lights if they're nice and close.