Sometimes, convenience wins, as when I want to work 'on-site'. It's much easier to transport a pad of paper than a pile of panels.
It sometimes seems that it takes longer to prep the surface than to paint the painting. Which means that working on paper removes the pressure to live up to the surface. Which makes it therefore annoying when a painting done on paper works better then a larger version done on a more prepared surface. On the plus side, with no investment having been made in prepping a canvas or panel, it's much easier to let go when something isn't working- or cut to the chase when it is.
I want to call these paintings studies- but then, every painting should be a study in that you should learn something from having made it, so they are, in the end, no less or more 'final versions' than paintings made on any other surface.