In short: Proton Calendar mobile app support for alarms in addition to just notifications or email.

If you have vibrate profile on, there is no sound with notifications from the calendar app.
Now alarms (from the clock app for example) will always use sound, even if you have "only vibrate" on.
I am used to setting events in advance and creating multiple alarms so I am not late for or miss anything important. It is not practical to use the clock app for that. Samsung (and I think Google) calendar support this.