Ravens are scavengers so if there was any food outside the ark, they could find food to survive. Doves do not stay in unclean places, so they return. The dove came back with a fresh olive leaf in her mouth, which indicated that there would be food for Noah and his family to eat.
So basically, it goes like this. If the raven came back, no food to be found. Raven did not come back, so send the dove to see if there is food for her. Dove came back with food, so Noah and his family will be able to find food.
