For Windows 10 integration
The idea behind having cortana available on iOS and Android is the dream of having seamless voice activation between all your devices. That is the utopia that Microsoft wants to achieve so that more advertising revenue comes it's way.
Google makes money from android by selling you (the apps you use, the sites you visit, the words you type in e-mails) and recently Apple and Microsoft have wanted a piece of that pie too.
Microsoft has this idea where you saying "hey cortana I need to visit shop x at 12pm tomorrow, remind me on my phone and give me directions" on your PC could then seamlessly have cortana on your phone do all the same things.
Likewise they want "hey cortana remind me to send an e-mail and produce a sales report for Joe in Excel using the jo-report-template next time on my PC" is something someone could say and all those things would happen.
My imagination isn't quite there with this, apparently there is pretty cool integration where you could do all sorts of stuff with it but you get the idea.