At pre-K, and Kinder age, for good or for ill, they do a lot of songs with gestures. So, if ones child is enrolled in such a program, and he can't mimic gestures. it is a problem b/c he stands out. It is not exactly self-esteem boosting stuff even if kids are not mean about it. They know they can't follow along.
Also, out in the NT world, understanding body language is important receptively because people use it, and being able to imitate handshakes and fist-bumps or whatever the kids are doing is part of the language. I don't think it ought to be important, but it is.
I am terrible at it myself, so I am not a great teacher with the subtle social ones. I taught "thumbs up and down", "high-five" and the "high-five ---down-low" one and I don't even know if anyone does that anymore. (Well, I have seen adults initiate it, so that is why I taught it. I don't think kids do it these days or think it is cool.
I do agree that the gestures that go with songs is a good idea. Even though my son had a lot of difficulty with them, he did enjoy them and want to follow along.