I do this with my dog. It also teaches her that she needs to be CALM before getting things. She has to sit to get her food, and sit before we go outside for walks. She's gotten so used to it that she automatically sits before I even say it
.
The other good thing is that she listens to the command (sit) more now! Before she would only do it if she knew she was getting something (food, treats, a walk, etc.) but now she'll sit if I say it, even if I have nothing.