When I had subwoofers in my system the biggest reason for moving them in any direction was to make the response as even as possible in their output band.
through measuring and listening you find that if they are too close to a wall or other boundary the response might be peaked and have an overly dominant note or frequency.
moving them away from the bounday is a way to reduce the peak and make the sound more balanced.
raising the subwoofers off the floor could be a way to make the response more even, especially if your room and furnishings put limits on how much you can move them away from vertical boundaries. remember, the floor is a bass reinforcement boundary.