Yes they are primarily a weight gain item, however, I have used them with dogs that have a high Metabolism.
Weda is one like this, although I've been lucky enough that just adding some 4% fat cottage cheese to her kibble made up for it. Instead of having to go full fledged satin balls.
It's something good to keep around for showing as well.
I knew people who would show their dog at a very toned weight, but a couple weeks later would want to show the dog with a couple more pounds, and would use the satin balls to get the weight up quickly.
Not something for every day, but a good item to keep for certain occasions.