If you are targeting multiple platforms, that's not always an option. PhoneGap gives you the option to target 10 platforms w/ an HTML/JS codebase. Native will always yield better performance and have best access to device APIs, but will also be more expensive to develop & maintain. PhoneGap gives broader reach at lower cost.
"So wait, you are telling me, I can build native mobile apps with just css and js"
-"Yes, all you need to do is learn these 1000 quirks and edge cases for performance, and learn/memorise our API"