You can train and optimize your negotiation skills all you want, but in my opinion it is extremely valuable to separate the "talent" from the "money". Top performers, no matter in what industry, are usually extremely passionated about their craft and undervalue the monetary value they can bring to the table.
Having a third party represent them puts a "neutral" monetary evaluation on them. Good management allows a much more strategic long-term development that focuses on value building and not just opportunity hopping.