Count me as one. It's nice to have a single local application that is set up for around 5 different accounts on two different providers.
I also like the immediacy of search on the local data. When I search for something I don't want to see a spinning busy-beachball indicator.
I use it to follow three Gmail accounts in parallel, since the web version is a PITA to deal with that scenario. Getting access to my local archive is a bonus point.
Whether this is true or not depends a lot on which the bubble is that you live in.
Well, now I've heard everything. This is either peak greybeard creativity, or that was a thing in like 1992 and a system has been left alone for 30+ years to just do its 90s thing. Either way I kind of love it.
On Android I use Fastmail's mobile client, but I'm thinking of trying the new mobile Thunderbird there too.
With that feature I could also help an elderly friend after Microsoft abruptly replaced the easy to use Windows Mail with a mess that they didn't even bother to translate into other languages.
A good desktop client, once configured, works a lot better than web-based email clients, especially (but not only) when you have different email accounts that you want to use in the same interface.
Your comments reveal a major blind spot. 99% of people (or whatever) are using dedicated email clients instead of webmail. They do everything on their phone.