I'm not a lawyer, but as I understand US courts from reading Groklaw and HN and Wikipedia, the arguments put forward in friend-of-the-court briefs can still be helpful to judges who may want to rule a certain way, but don't know all of the possible arguments they could use.

There's only so much time they and their staff have to do research, so members of an industry and their legal representatives can contribute if the judges are on the fence or just looking for arguments.