I purposely give them dirt from the run about Day 3 in the brooder. To me, this has several advantages. They get grit to help set their system up the way it is supposed to work. I know they don't need grit if the feed is all they are eat, but I like them to be ready if a hard shelled bug crawls into the brooder or they eat some bedding material.
They get probiotics from the adult chickens. There are a lot of examples of this in the animal kingdom. For example, a baby elephant will eat some of its mother's poop when it starts switching from mother's milk to vegetative stuff in its diet so it has the right bugs in its intestines to digest what it's eating.
Another reason I feed them dirt from the run is to get the protozoa that causes Coccidiosis into their system at a very young age when they are best able to develop an immunity to it. I don't feed medicated feed but just keep the brooder pretty dry so the risk form Cocci is really reduced. I still closely watch them for possible Cocci problems, but I've never seen any, either in the brooder or when they hit the ground.