Cortana on iOS and Android will not be able to do things which will be possible on Windows Phone. But still, users will be able to perform search, get reminders and alerts. The Cortana companion app will be available for Android in late June and for iOS later this year.
Cortana will compete with Google Now on Android and Siri on iOS. Google Now and Siri both are popular enough. Still, Microsoft want to try its luck on these platforms. Once the Cortana is available, I will surely try it to see how it works on other platforms.
Microsoft is trying to make its Windows 10 OS compatible with other popular platforms to help users work on cross-platform without any issue. This integration will surely help Windows 10 users easily access their files from Android and iOS devices with Phone companion app.
Source: Windows Blog
