So much can be learned about this candidate in the response you receive. If a counteroffer is expected then you want to know why, what the candidate will do, etc. This could impact the structure of your offer. If the candidate expects to be shown the door then you know they could be available sooner to start. If the reaction anticipated is that of relief by the current employer then we need to know why. Overall you benefit greatly by learning how things could play out as it will shape your decision to make an offer, your decision on compensation and even your decision on when to expect this new hire to arrive.