Out of curiosity, what makes people prefer desktop apps for mail over just visiting the browser version? The idea of downloading a desktop app for something I can do in the browser makes no sense to me. At least on mobile, the ergonomics of apps tend to make the experience better than the mobile-web version.
Less tab clutter, enhanced ability to use OS app/window management facilities, no space eating browser chrome, works without a browser being open (yes that does happen on occasion).
It doesn’t apply to Proton’s desktop app specifically but generic email clients also allow management of multiple accounts in a single window without the messiness of forwarding.