I mainly use it in procreate (I would guess it works the same way in Photoshop) to paint inside a layer, without using a selection. In 3D, i would like to paint a layer or a selection as a 3D canevas. The problem with the repaint tool is that it repaints the entire line/3D hull. And painting with a brush is harder to control and gives a 3D effect. In other words, using a hull as a guide and " 2D painting" over it.