When you are a foster parent, your purpose is to help that animal get ready to be adopted into a forever home. Animals that are fostered often need more help than the shelter can give them, behaviorally, for health reasons, or many others. It's your job to train and work with the animal so that they know how to be a part of a loving permanent home. Being an owner, you have promised that animal to be there for them forever. You can't (unless you treat pets as possessions) give them away. With foster animals, you can't get too attached unless you want to end up adopting every animal you try to foster, which I am notorious for doing.