Some folks feel the need to let them "cook" for a second or two if they are tossing them into a place where they may have someone toss them back out...
Or, conversely, if they're trying for an airburst.
Personally, even with just practice grenades, I didn't like the whole concept.
We had one target which was supposed to be several Ivans... I hit the center one in the chest with the grenade, and the instructor told me that if I'd missed, it would have gone out of the impact area. So I asked him for another chance, called my target, and hit one of the kneeling Ivans. Low.