Regions have overflowing animal shelters filled with dogs in need of homes. Many are euthanized because there is simply no place for them to go. For rescue organizations, foster owners are one of their most valuable resources. Rescue groups often lack large facilities that can house enough homeless pets. They often need foster families just as much as they need donations (or even more). Of course, if you cannot foster a pet, your donation will still do plenty of good.
If you work long days, fostering a puppy might not be the best thing for you or the puppy.
If you cannot provide a lot of exercise (due to time or physical constraints) don’t foster a high-energy dog or puppy. A calm adult or senior dog might be best.
If you don’t enjoy training, don’t foster a dog who needs a lot of one-on-one training or socialization.
A large or rambunctious dog might not be best if you have a small home with no yard. This also may be problematic if your current family dog is older or intolerant of high-energy dogs.
If you don’t have reliable transportation, you might not be able to foster a dog who needs to be transported to many places (such as adoption events or the vet). A dog that has health issues and needs to see the vet frequently may not be the best choice for you (although some organizations have people to help with transportation).