I would like to know why agents would like to know whether they are seeing your manuscript exclusively. Does it make a difference in making their decision about your work?
It does not make a difference in an agent’s overall decision, but letting an agent know she has an exclusive gives her breathing room. If in fact you have given an exclusive (even voluntarily), she now knows there’s not a huge rush to get to it. In other words, it can easily go to the bottom of her reading pile since there’s no real competition. She also assumes that if she offers there’s not going to be any worry about whether or not you’ll accept. Of course you’ll accept because, again, there’s no competition.
It’s also a matter of curiosity. I know that if I find out that other agents, or editors, have also requested a full manuscript, the material has already received some sort of stamp of approval and might pique my interest more.