Recently, at my job at a bridal store, a woman and her daughter came in a beautiful golden retriever, who had an aid's vest on. She explained that they were fostering him until he was grown, after which he would be trained as a therapy dog for kids through a program called Four Paws.
This is actually very common. Many therapy dog organizations can only begin the training process after their dogs are fully grown out of the "puppy stage." Many organizations (healthily!) breed puppies or have a relationship with a breeder, then foster out the puppies in order to learn their temperament and get them house-trained before aid-training them.
So why should you consider fostering a dog? The benefits are high. If you've just lost a dog and don't feel emotionally ready to commit to owning a new furry friend, fostering can help warm you up to the idea of buying or adopting again. Whether you're fostering an animal for an adoption agency or for a therapy service, the dog will only be with you a certain amount of time. This will allow you time to heal and form bonds with other dogs, while not emotionally investing in a dog's lifetime (at least not completely.)
Typically, adoption agencies and service dog programs will pay for all of your foster dog's cost. Vet bills, food, and any other financial costs will be covered by who you foster from.
Fostering also gives you vital skills and knowledge about how to raise a puppy, which you will definitely need to raise any puppies you might get int he future.
Aside from this, it builds your patience, tolerance and opens your heart! The downside to fostering, of course, is that it's inevitable that you will form bonds with your foster dogs and then not be able to keep them. But the best part of it is that you get to see them go somewhere awesome, especially if you're fostering a service dog. You get to see your dog make an adult or child not just happier, but healthier!
There are service dog programs everywhere that have foster programs as well. You can find these simply by searching "service dog foster programs" in your area. Open your heart, it'll make you and your foster dog feel warm and fuzzy!