Some people are able to arrange for family or friends to care for their kids in the interim. Or, work an alternate schedule temporarily, until the space opens. I also keep people's names on file but have never yet called and had someone take a spot that is offered. Everyone has found care.
I think it's also kind of a nicey-nice way for providers to "let people down" a little easier. As in, tell them that you're sorry, you have filled the space, but you will put their name on your waiting list.