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.
Local search in most commercial software has regressed since ~2000.
My only explanation is that local search is inherently cross-team and integration-heavy. Consequently, if there's no higher-org prioritization it just molds and breaks over time, as unaddressed integration bugs pile up that cannot be fixed by a single team.
Companies shipping their dysfunctional org chart.
Still, I never thought I'd say that I could do things with the built-in search 20 years ago that I can't today.
The only ads I ever see in either Google or Proton web mail clients are for their own services, or if I check the spam folder. And I don't have an ad blocker set up (I have NoScript, but there isn't any third-party JavaScript anyway).