I'm not the OP, but I can think of a few reasons:
--broody provides warmth for the chicks, which might be needed if the weather is really cold, or if the chicks are feathering slowly
--introducing the chicks to the rest of the flock (The broody could make this easier or harder, depending on the coop setup and how the other chickens react to the chicks.)
--makes the hen quit being broody
--person enjoys seeing hen with chicks
Of course, all of those things can be accomplished in other ways too. But I don't see any problem with having the broody hen adopt older chicks, if the person wants to, and if the hen & chicks cooperate.