Another use case is someone like me. I absolutely love Fastmail's web client, it's better than any other web client I've used including Gmail. However, I also have a few email addresses for domains that I own, and I don't want to deal with clunky forwarding; I prefer to use them as-is. For those it's much, much easier to use a good desktop client rather than set up and maintain Roundcube or even SquirrelMail on my servers.
Windows 10's Mail app is not quite up to the task; it's fine for Outlook.com webmail but I need something much more powerful on Windows without having to buy Outlook proper (on Linux this obviously isn't an issue, there is a wealth of good desktop email clients for X and the console). So far I've been doing well with Thunderbird, but I'm tempted to give Vivaldi a shot since I was always a fan of Opera's email integration.
Windows 10's Mail app is not quite up to the task; it's fine for Outlook.com webmail but I need something much more powerful on Windows without having to buy Outlook proper (on Linux this obviously isn't an issue, there is a wealth of good desktop email clients for X and the console). So far I've been doing well with Thunderbird, but I'm tempted to give Vivaldi a shot since I was always a fan of Opera's email integration.