When using detergents and possibly some other detergent free formulae, but not microsplitters, a pile brush is used primarily to distribute the solutions throughout the pile. These solutions then need dwell time for the soil to be broken down into something that can be rinsed away.
High levels of mechanical agitation will obviously accelerate this soil breakdown as well as opening the pile, lifting out fibrous waste and speeding up the job. Remember T.A.C.T or C.H.A.T.? Reduce one element and you have to make up with another.
A CRB machine in particular offers benefits beyond the chemical side of the cleaning process.
Safe and happy cleaning

Ken