You want to put your best foot forward, so you wear your A-Listers.
There is also B-List undies (you wear them out to birthday parties, family get togethers, etc.) when there is a slim chance of any hookups.
Then there is the C-List undies. These are the ragtag bunch of ginch that are hanging on by the thread, and half the waistband is misplaced. These are usually worn around the house, when the chances of a hookup are slim to none.