The juice box flaps are there to make the box handier
Although they may look cute, the flaps at the top of juice boxes are not meant to make the juice boxes fly. Many people may be unaware of the function of the triangular flaps on the corners of these little boxes. And the following explanation will make total sense when you think about who the intended customer is.
The purpose of these flaps is to make the box handier and help children to hold the paper box properly. Moreover, it is quite common for kids to stain their clothes while drinking any liquid and these stains are not easy to get rid of, so another purpose of these flaps is to stop kids from squirting juice everywhere.