I have fostered in the past for a rescue. Depending on the situation, it can also free up more space in the rescue to take in birds that are in terrible situations. The rescue where I volunteer (used to be very active volunteer until my life got much more complicated) always had a waiting list. We tried to use foster as a last resort (as we have a facility and prefer for the birds to be at the facility so potential adopters could meet them) but when we were in a smaller location, I took in a few in order to make room for some really bad abuse cases.