Quality varies depending on who's on the account
The accounts your senior team runs tend to produce the work the agency is known for. The accounts run by mid-level staff are usually solid, but the standard isn't quite as consistent and clients tend to notice that gap long before they raise it.
The methodology your senior team uses becomes a workflow the wider team can follow, with the SOPs and examples that show what good looks like at each step. Quality becomes a property of the process, not a property of the person running it, and mid-level work moves closer to the standard the agency was built on.