Front-end frameworks are going to give you a few things: more control over routing, more "app" like behavior, instant state changes (easily done), and most of all: your front end isn't going to be DOM driven. Lets say you're tasked with redesigning an admin area and one really ugly area are forms. Rather than having the logic and styling bound to classes, that functionality is bound to a component. So, for large front-end applications the maintainability can be tremendous. I would say the big 3: maintainability, app like routing, and instantaneous state changes (across diff portions of your app) without doing much work.
With Rails 5, you get the benefit of both. You're not doing your backend in javascript, but it plays nicely with modern front-end libraries and frameworks.
With Rails 5, you get the benefit of both. You're not doing your backend in javascript, but it plays nicely with modern front-end libraries and frameworks.