They are also the reason that partnerships still thrive. A couple of years ago I was talking with this (big law) partner at a dinner, and we came in on the topic around partnership - and he just said it straight: The competition / rat race towards partnership is 100% skewed to the firms advantage. They get dozens of highly motivated, highly competent professionals competing against each others to prove their worth. They'll be on call 24/7/365 to show that they're worthy of partnership. Even if they become partners, most won't be rainmakers - and many would have made the same amount of money if they struck out on their own, opening a small boutique firm.
Prestige is a powerful motivator.