Because of what I said right after that I called it a stupid reason?
If Foden performs 5/10 on the left and 8/10 on the right while Saka, since he's brilliantly adaptable, performs 9/10 on the right and 8/10 on the left, you'd want to switch them.
These numbers are obviously just to illustrate that you don't go by who's the better player, but by which option will make the team better overall.