There are two reasons:
2. You want to create an application that will target multiple mobile platforms and you want to only write the code once.
It's this second option which PhoneGap was created for.
And for balance, here are some reasons for not using PhoneGap to create a Windows Phone 7 application:
- You want to use some functionality which isn't available in the PhoneGap implmentation (and plug in support doesn't exist yet)
- You want to use the power of Silverlight and XAML for creating your UI.
- You want to use the power of XNA to create rich 3D graphics.
- You want the benefits of using Visual Studio and Expression Blend.