Any code involving a mid- to large sized web application that needs reactivity. Templating engines like handlebars are used on the server side, react and vue belong on the frontend (most of the time, but not exclusively). That's the reason people use them: If you want to, you can have your logic executed on the client-side only, and host whatever else you need on a cloud-provider. Client-Side computing doesn't scale particulary well when your userbase grows fast enough.