since the plane is in 3D, your view of it changes as you rotate the view perspective. It will also render, since it is “there”. With a background image, that image 1) is locked to the view; you rotate view you lose sight of the image, and 2) it doesn’t render. So for modeling, I like background image so I can quickly render. of course now you can set the plane to not render using the outliner, so it’s not that big of a difference anymore.
The nice thing about planes is that if you have a non-orthographic picture of the thing you are trying to model, you can set it to match; whereas with background images, that pic would be useless.