Yeah and sometime you may have a situation where waiting for a vet might not be humane for the snake. Some vets won't take reptiles, or it might be after-hours or the weekend when you can't find an open vet, or an emergency vet that will take in reptiles.
In those cases, having something predetermined can really help both you and the snake, you for not making it quite as traumatic because you've considered it, and the snake for not having as much time to suffer until you decide.