The manual setting has varied purposes, all pretty important.
The obvious use is to level the coach manually if the automatic selection doesn’t do it to your satisfaction. The automatic feature gets it close, but manually you can fine tune it. You can also bias the leveling to your preference. If we have a choice we want the front as low as possible so the first step is not so high. The automatic feature may leave it at the road height and then raise the rear.
When the manual position is vital is it can eliminate dragging. If you cross a high crowned railroad track you can raise the entire coach before proceding to prevent getting hung up or dragging. Some driveways have a very steep approach angle so raising the front or rear could prevent dragging or damage to the hitch or front bumper valance.
If you have a height clearance issue lowering the coach can give you as much as about 4″ of lower height. Raising the front can prevent damage if the front has to swing over a curb such as when making a K turn on a street.
I believe all Prevost coaches with level low have the ability for the driver to manually adjust the coach height.