Many cross platform technologies are available in the market like PhoneGap, Appcelerator etc. Using which you can create cross-platform applications and with a lesser cost than developing the Native application.
Xamarin is a unique technology in which addresses the issue flexibility to such Hybrid technologies mentioned above and cost associated with creating the native application.
With Xamarin, create the native application for your business/product/idea with deploying just single resource. Xamarin helps create Cross-platform native applications for iOS, Android and Windows using only one tool. This is what makes Xamarin special and gives upper hand.
Thus, investing in only one technology gives you output for 3 technologies with the same advantages as you get in creating native mobile applications.