It's not moving focus away from Linux/Office365 but it accommodates people that want to have their Gmail accounts in the same client as their Office 365 / Exchange accounts.
I'm a consultant. My clients often use Outlook/Exchange. My consulting company uses Google Apps. I also tend to create a per-client GMail persona, so that I can work around issues with corporate Exchange servers (aggressive content filtering, misguided attachment quarantines, or just to keep work-related-but-social stuff out of the corporate inbox), and have a convenient OAuth login option. I would welcome anything that allows me to use a single client.
Another reason for wanting a client that works with Exchange and Google is simply wanting to use the same app both at work and at home, even if you never cross the streams and access them both from the same instance.
And as others have mentioned, Linux email clients suck. I'm a huge proponent of open-source software, and prefer free clients wherever possible. No one has made a free email client that I can tolerate. They're so bad that I tend to stick to web clients. Hiri's client reminds me of Polymail on MacOS, which is the only non-game, non-cloud application I've paid for in the past decade.
Another reason for wanting a client that works with Exchange and Google is simply wanting to use the same app both at work and at home, even if you never cross the streams and access them both from the same instance.
And as others have mentioned, Linux email clients suck. I'm a huge proponent of open-source software, and prefer free clients wherever possible. No one has made a free email client that I can tolerate. They're so bad that I tend to stick to web clients. Hiri's client reminds me of Polymail on MacOS, which is the only non-game, non-cloud application I've paid for in the past decade.