I understand why publishers often ask for exclusive submissions. They're counting on a story that has been submitted to them to be available for them to purchase if they decide they like it and it fits in with their current publication.
But why would an agent request an exclusive submission when it's not so much the story they're looking at but the author themselves? Is this a common thing among agents? Generally my understanding is that authors work for publishers and agents work for authors. I understand it's more complicated than that, but what advantage does an agent gain by requesting exclusive submissions other than keeping an author from submitting to other agents until they decide to reject or accept them?