I have worked in shelters for years, and fostering is a great program, but it can be sad and hard too.
I fostered seven cats from my shelter, and ended up keeping number 7! I recently took on a kitten who needed a leg amputation so that I could rehab her.

Whether or not you get a dog or cat, it can help you immensly. Maybe it you let them know what you are able to do, they could set you up with someone that it appropriate. Or, some shelters have programs where you can pick them up when they close for the day, and return them to the shelter during the day so that they can be visible for adoption.

Dog sitting for friends/family can also be a slow way to get into doing this type of thing. If you see it more as a temporary job as opposed to forever, it can help when they go on to their new homes.
Regardless, it is one of those things where you get the love, and companionship and they get to have a break from the shelter and that time can help them get adopted quicker.

Good luck!