The idea for this feature came up when I was working on an anthopomorphic figure made out of parts on different layers. I was adjusting the angles of the arms and legs by eye but, every time I rotated a shape, it rotated about its centre so I had to adjust its position after rotation.
That made me think how useful it would be if I could set the centre of rotation of the arms as to top of the arm where it joins the shoulder. That way I can just play with the angle of the arm without all that mucking about in x and y.
I was reminded of it when replying to a post about stacking images of the night sky where you could align the images by the brightest star, set that as the centre of rotation on all the layers and more easily get a useful stack of images.
Here's hoping that you at Pixelmator HQ like this idea as much as I do.
P.S. I mentioned resizing because, then resizing using the fields in the Arrange tool, it would make sense of the layer to resize around its centre of rotation.
What features would you like to see in Pixelmator Pro?