What is the distance scale window on a lens for?

That is a focusing distance indicator. It shows how far away the lens has focused, from the nearest to farthest. It will typically show the distance in both feet and metres, along with the infinity symbol that's shown in the photo you've included.

It can be useful when you know a fixed distance that you manually want to focus on, or if you want to know where the lens is focused within its range, such as for close-up photography. It's very useful on lenses that are designed for manual focus, where it will be considerably larger and more detailed than here, but isn't an important feature on a contemporary auto-focusing zoom lens at this price point. Here its small size and lack of precise markings limits its function to giving you a secondary visual indication of AF function, and as a way of confirming when the lens has reached its minimum focusing distance.