The main device type for UWP is any Windows 10 device. See the list of benefits mentioned in the link above, which extend beyond phones. I've never owned a Windows Phone, yet I always prefer UWP apps.

Also there is a big market for 2-in-1 tablets, and new small Windows on ARM devices are coming out very soon.

http://www.zdnet.com/article/microsoft-to-pc-makers-lets-mak...

Not every kind of client app is suitable for the browser, especially if you care about performance and deep native platform integration. And I'd rather not deal with JS frameworks when not targeting the web.