Why phone gap is required to customize HWC application?

Hello,

My friend asked me to use phone gap to customize the HWC application.I'm bit confused, why should I use phone gap(another 3rd party tool)  for this?

Can I not customize the HWC application from "Generated workflow-> Project name -> html -> (css and js)" ??

How phone gap is going to give benefit for customization? Will it give me auto generated code(js,css and html) from the UI??

Please help me to understand.

Thanks.