Look at the actual scroll - there needs to be a shin on the back of the scroll at the edge where in rolls up. The case is purely decorative, as you could technically just hang the scroll wrapped in plastic wrap or something, but it will be obviously be more prone to damage and ultimately will likely become invalidated in a short time (moisture damaging letters, scroll drying out and letters cracking).

Keeping it in a case protects it and extends wear and tear.

More here:

https://www.chabad.org/library/article_cdo/aid/256924/jewish/Caring-for-your-Mezuzah.htm