Why release Cortana for iOS and Android?

This morning I had been reading about rumours that Cortana, Microsoft's digital assistant for Windows Phone, is to be released as an app for iOS and Android. This decision didn't seem to make much sense to me until I found this article from Gregg Keizer at Computerworld:

"This will reinforce the value of Windows 10," argued Dawson, referring to the Cortana-equipped upgrade scheduled to ship later this year. "Microsoft wants customers to have an integrated experience, a complete experience across all devices. But although Cortana is a headline feature on Windows Phone, the most common scenario will be a user who owns a Windows PC with a non-Windows phone."

When I was contemplating the impending release, I hadn't realised that Cortana was in the pipeline for Windows 10. It makes much more sense now. Microsoft is working on some interesting things at the moment and this could become one of them.