At current, the Proton mobile app is very lacking in features that made its competitors much more useful. One such feature is the ability to bookmark specific folders saved in the Drive that can be quickly accessed from the first screen you are presented with when opening the app.
At the moment, when navigating through file folders in the app, it seems like half the folders don't even seem to load. I do not know why this happens, but it's persisted through multiple devices running the latest version of the app, and it makes getting to frequently sought-for folders frustrating without some kind of shortcut system. The proposed idea would circumvent this issue at least when returning to frequently needed directories.