None - they're just required to ask if you're on the dining plan, and if you're at a DVC resort, you're probably a DVC member so you can't get the plan. Most ask "Are you a DVC member?" and avoid the whole dining plan question alltogether - at the disgust of Disney.
Asking serves minimal purpose. Yes, Disney can get a pretty printout of all the people booking with dining plans. With prepaid events, a dining plan scores you a credit card guarantee instead - so its mildly helpful for that.
Otherwise, Disney sees it as a selling point in 2 ways. If you have a dining plan, you need to make all your table service ressies. Disney says so. (It's really not a bad idea - better than losing out because they're sold out, which happens more frequently now)
2nd, if you don't have the dining plan, why not? If you're DVC - the answer is you can't have it. If you're off property - Why are you off property? Haven't you heard how wonderful Disney Resorts are...?
See? The mind of a marketing machine. So, in short, they're required to ask you. It has nothing to do with availability and the ressie is marked as a reference as a courtesy to the restaurant though you're under no obligation to use the dining plan as payment.
I do think it helped the World Showcase restaurants come around to accept the plan (which they originally didn't) because the numbers showed they were losing ressies.
Anyways, just a number to crunch. Disney's all about data.