The one huge benefit is speed of development. By using HTML5 (and maybe CSS and JavaScript) you can build a web app,
test and debug it in a web browser, and then turn it into a standalone app.
It won't be as fast as a native app, but if it's not processor inventive (for example, an interactive eBook), then it's plenty fast enough and you saved having to code it all in
Java (or Objective C for iOS) - unless, of course, you prefer to code that way.