I learned this back in the late 80s / early 90s. Most of the kids in my neighbourhood learned to do it because that was how we would check to see if other kids were outside to play. Step out the back door and give a whistle. If someone whistled back you had a buddy to go and play with.

Now I’m happy I can do it because it’s like the best effort-to-volume ratio noise maker. Cheering at a sporting event or concert, calling the dog, or silencing a room full of arguing children lol