something that I often run into is that it is quite difficult to comunicate wireframes to clients.
tools such as figma and XD have been what I have been working with.
Having some basic prototyping tool in Mendix might make it easy for clients to communicate ideas
I'm not sure about the level of styling you would or wouldnt want, but definitly not having to make a domain model and micro flows would be nice (list views with dummy data)
also the ability to have some kind of wireframe looking css might be enough.
This could also serve as a base for the screen developers start working on.
something similar as SAP does with https://www.build.me/ could be useful for that purpose. especially if it has a function to use it as base template for your real mendix project