If you intend to stop using Protonmail, non of this is required however if you just want to clear down unwanted emails and claim back storage quota this may be of use.
According to ProtonMail support in normal operation you need to move emails to the delete folder then go into the delete folder and delete all to get rid of them.
This way youre 'all mail' folder wont just fill up eternally, increasing storage and thus require to buy more storage
Also if youve got a paid plan and wish to delete an associated domain name, you can't do this untill all emails that are addressed to that domain have first been deleted.