But yes, it's a shortcut - if you select an entire surface object instead of selecting curves you can then run Planar and it will find sets of open edges that make planes, then construct the planes there and join them in to seal them off, so it combines a few steps doing it that way.
Hi Pilou - you don't need to select any individual faces at all actually, you just select the whole object with just one click so the whole thing is selected and then run Construct > Planar. Any planar openings will get sealed up.