When I go into character-wise visual selection mode, one end of the visual selection is fixed while I move my cursor around to define the other end of it.
Let's say I've arranged a visual selection that encompasses several lines of my file. And then I realize that the fixed front-end of my visual selection is off by a bit. Maybe I've selected an entire function definition and I just want to inner part of the function.
Instead of starting over with my visual selection. I can leave the right-end of
the visual selection where it is, hit o which will switch the moving end to
the other side, and then continue making adjustments from there.
I can always hit o again to switch it back to the original side.
See :h v_o for more details.