MUI is not only about good looking windows but it's a framework to ease GUI development. It is then needed for some apps the biggest one being Odyssey.
If devs want to build multiplatform apps they use MUI as the official GUI framework for OS4 (Reaction) is not available elsewhere. (except AmigaOS 3.5 and up and I'm not sure it include all the latest OS4 classes)
Kamelito
