You have enormous power over people who have next to no power. You can make a nobody somebody (sort of). But boy can that power backfire.
Studios and production companies want to trust you, so that means they'll work with your client. However, if you hold one of your stable of entertainers out as being the next classy yet down-to-earth film star (JLaw, anybody?) and they turn out to be the next tabloid flame-out instead (remember Tara Reid) your reputation takes a hit.
The next time you pound the table for a new client who really does have talent, your poundings may fall on deaf ears. So there's a balance here for how you can use and abuse your power over nobodies.
But over people who have already "made it," you more or less have no power. Or at least very little. What's an agent going to do to push around Brad Pitt or Steven Spielberg or the guy who did Family Guy? Not much. What you have instead is power over studios who want to use your superstar client. Oh how the tables have turned.
There are two really, really big agencies in Los Angeles: Creative Artists Agency (CAA) and WME Entertainment. They have direct lines into the biggest studios.
Then there are a few big, but not that big groups like ICM and United Talent Artists (UTA). They'll get on the line, but they may have to hold for a couple minutes. Then there are the lower level boutique agencies—whether studios take their call really depends on who the talent is or whether or not the executive gets invited to the head agent's daughter's birthday party.
Your power all depends on where you fit in this hierarchy. You might own your own commercial agency with offices in L.A., New York, and Toronto, but some snot-nosed brat right out of junior agent school is still getting more clients booked because of the three little letters he works for (which makes you S-A-D).