Working offline such as on a train. This includes having all emails and attachments saved locally. Also useful for backups. E.g. suppose you get locked out of your email account because "AI says you look suspicious". How ruined is your day?

Managing multiple email accounts in a single interface.

Easily moving emails between different accounts and from online to offline.

Relatedly, storing years of emails with attachments on a local drive is cheap. Storing them in webmail is a hefty subscription fee. I have no issue with one time payments, but I like to minimize subscriptions.