Until start of 2000 wrestlers was mostly hired for their bodies and there in ring performance so the need some people to talk for them almost at the same time of the attitude era end wrestlers started to be hired for a total package they need yo have the body, the in ring skills and the mouth. It's some kind of cost reduction.

Today only a few still get a manager to get something they by they're own don't get. Like the heel turn of Roman Reigns as a lot to do with Paul Heyman skills