These locations just hold the vaccine in refrigeration for a few days, I guess. They are mostly doing the Moderna vaccines, from what I can gather. There isn't that kind of urgency. They aren't putting a call out to give out doses that are going to expire or anything like that. They are advertising available appointments as soon as same day or next day.
Our county health department tweets several times a day to the effect of "if you are not in an eligible vaccine tier, do NOT make an appointment and do NOT under any circumstances show up without an appointment. You will NOT be given a vaccine unless you are eligible and have an appointment."