It's for quality and personalization, not for ads. This is Terry Myerson's blog explanation of data usage.

https://blogs.windows.com/windowsexperience/2015/09/28/priva...

That assumes you believe Microsoft, that they are not deliberately lying to their customers. Not plausible? Microsoft claimed Skype has end-to-end encryption, too.

http://arstechnica.com/tech-policy/2014/12/newly-published-n...

https://www.theguardian.com/world/2013/jul/11/microsoft-nsa-...