Even though I only have them for a few weeks or months, I get them for the very cutest time of their whole lives! Once they are gangly "tweens" they are more trouble (climbing curtains, knocking things off shelves). Then they are adults, and although nice--I have two of my own--they are pretty much the same from day to day. How else can you have such an endless supply of kitten-ness besides fostering? The season runs from late March to sometime in the fall--cats can have second litters at the end of summer. So winters are mostly free.
What I give is taking responsibility for them, spending the time to care for and socialize (litter box, feeding, laundry, dishes, meds given when needed). What I get is the fun of nurturing and watching them grow, solving little problems for their care, and the sense that I'm giving something back to the community. The people I work with at the shelter are a great bunch of dedicated animal lovers, so I have friendships around our common interest. I do volunteer there once a week in addition to fostering. I did that first, then after almost two years someone encouraged me to give fostering a "try", and I was hooked.