Were these people that were arrested not using a VPN?

Ah okay, my bad. I misunderstood. I guess to be able to answer your opposite question I would have to try thunderbird myself and see if I still prefer web based or not. Otherwise I don't have an answer to why I use web client over thunderbird other than it's already there and ready when I set up my account.

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.

ELI5 please, why would I use thunderbird over a web client? I have used a local email client in years but it seems everyone uses and loves thunderbird.

How does this affect people self hosting an encrypted chat service? Would those people be at risk of a police raid or something?