When someone states their request outright the recipient may be "conditionally there" based on the content of the request: They'll reply if they find the question interesting/relevant and worth interrupting what they're doing and put it off otherwise.
The asker, however, would prefer their question get answered. If they first determine if the recipient is present there will be social pressure to answer the following question, even if its a waste of the recipient's time at the moment.
The recipient may find it aggravating to have people soft-force them to pre-commit to responding when they don't even know what will be asked.