Any insight into the usefullness of making app versions?

So if you've got a web app, when does it make business sense to develop iOS / Android versions? I'm using rails w/ turbolinks and looking at https://github.com/turbolinks/turbolinks-ios which looks like it might not be that much development overhead.

But generically speaking, would an iOS + android version be a big boost? What indicators would you use to decide whether to pull the trigger on developing that?