Because then I can access the same client from anywhere, any platform without having to worry about learning the interface for several different clients.

You cant set up server side filters from any floss client for any platform afaik. If i am wrong someone plz tell me because it is the #1 problem every time. Always go back to webmail and proprietary mobile clients.

(Am talking about imap servers and generic clients. So not like proton etc.)

Most operating systems, excluding Windows, are shipping with decent native and fast email client.

Even Windows ships with one ("Mail"). I don't have any real experience using it (on my Windows work laptop I have Outlook, and on everything else I use Thunderbird), but it looks fine to me.

I have no idea what the pricing scheme is for Outlook these days, but Outlook does remain genuinely excellent too.

Doesn't windows ship with a native email client? I don't use Windows but I remember an email app on it.

Sounds like you're making an a point from privacy, which I agree with and subscribe to. But I feel like there's something in your point that is missed in that the email provider has the emails still. If I have a Gmail account then I doubt it matters if I use thunderbird or not. So that pushes me to use a more privacy focused email provider but if they are then their web client should also be privacy focused. So if total privacy is the focus then hosting your own email service would be what is needed. But if privacy isn't the point then convenience is more important. And going off the other replies to my question, the only reason is if you have multiple accounts and want them to be accessible from one place.

I have 2 main email accounts, 1 that is family and friends focused and 1 that is privacy/purchases/etc focused and I actually like them to be separated so thunderbird doesn't sound very useful for my case. Not sure if there's something I'm missing here but if there is then I am willing to read and learn. Especially when it comes to privacy focused stuff.