Why there is no Yammer App on Windows 10 ?
It's complicated to understand this situation.
Without App we can't share easily information from egde to yammer for example.
As we do on iOS or Android.
The Windows 10 app is listed in the roadmap below, from earlier in the year, so it should be on its way but no timescales currently:
A slight update, the app is now listed as an item on the Office 365 Roadmap as 'In development':
Yammer Desktop Client - Yammer will develop a desktop client for Windows 7, 10 and Mac.
No other information right now.