It doesn't do much good to ask your dog to Sit if she gets right back up again, especially if you're asking for the Sit to keep her still for a period of time. If you don't teach her that Sit means 'Sit until I tell you you're done', she will Sit until she is done, which will usually be ASAP.

Isn't that what the Stay command is for?

Stay does work to keep her still, but consider this scenario. Somehow, your dog gets out of the yard. She is on the other side of the street, and cars are passing by. She sees you in the yard, and starts to run towards you. If she does, she will almost certainly be struck by a car. You want to stop her in her tracks AS SOON AS POSSIBLE. When seconds count, you want to be able to say "SIT!" and have her sit and wait for you to come get her rather than having to say "SIT" pause "Stay".

An additional benefit of the Release command is that it clearly shows your dog that you are in charge because you are the one who decides when the exercise begins and when it ends.

How do I teach a Release command?

Do not allow your dog to end an exercise, such as Sit, Down, or Stay, until you give the release command. If she moves from position, do NOT repeat the command. Gently replace her, wait a few seconds, then give the release command. If your dog is having trouble waiting for release, give treats as long as she remains in position. Slowly start to space out how often you give the treats until you can phase them out altogether (except for the random surprise treats, of course!).

The Release command can be anything you choose. "Free Dog", "Okay", "All Done", "At Ease"... whatever you want, as long as you are consistent.