Some vets are more comfortable with having them stay the night to monitor them. But I am not sure if this is a 24 hour clinic? If it's not, that would concern me just having surgery and then put in a kennel. Ask if you can bring her home, it's really not neccesary for them to stay the night unless their spay or neuter was rough but it shouldn't be on her since she's a pup and probably would just be a normal routine spay. My dogs always came home the same night except Dixie.