downloading emails and storing them locally for offline reading, categorizing, searching and drafting. "Caching" usually just means if you opened the app with connection, it won't go bonkers and will probably let you finish your immediate task + some basic functionality if you lose it. Can't close the app though.
I turned my WiFi off and opened the app it was just a white screen. I suppose its beta still. But my dream is to keep a local copy of all my mail just got a cache.
To save myself the hassle of having to rebuild the electron app every once in a while? I'd rather not open my browser, go to their website and log in with 2fa every time I want to read an email.
The main benefit is since it is locally installed, it is harder for proton's server to access your encrypted data by serving you malicious JS. A malicious desktop app/update could be served too, but that may be trickier.