It made sense at the time.
Then I learned how to horseback ride. And I was told whenever the horse listened to a command, we were to say, "Good girl!"
Then I learned how to train a dog. And I was told to say, "Good job!" or "Good girl!" whenever the dog listened.
Know that I am not comparing my Little Olive to a horse but that I am comparing methods of teaching. Why on earth should I thank a horse but not my own offspring? We say "thank you" and give positive reinforcement to let a horse and a dog know they just did the right thing. We want that behavior to be repeated.
In my extremely knowledgeable 2.5 years experience of parenthood, I think this makes sense, and I've seen firsthand it makes a difference. For example, before nap time today, Little Olive was being very stubborn about putting away her toys. I repeated myself so many times and reworded my directions to give her clear steps: "Pick up the green block, and put it in the blue bin." Over and over and over again. She was testing me for sure. But finally --- she did it! I was very frustrated but managed to say, "Thank you. Now pick up the red block, and put it in the blue bin too." Her expression actually changed when she heard me saying, "thank you" - and she quickly listened this time around. I'm not saying I know all the answers, but I'm happy this works!