Developers used to call this kind of architecture headless, but it's called decoupled more often today (because who wants a "headless" website?!) It's the next cool word today. It's the today's black. There's a lot of talk about the decoupled in the Drupal community now. It's the eye of the hype now, being cool and technologically advanced. There can be reasons, however, why this hype may not at all be for you.
1. What Decoupling is.
What does the term decoupling mean? It means basically, that your Front End is divorced from your Back End. Traditionally, if you use a CMS, like Drupal or Wordpress, the CMS, the Content Management System, handles both the way how you manage your content, and how you serve and display it. Thus, a CMS will allow you to construct pages or data entities with fields of different kind, like text, images, media, links, addresses, etc., and then it will also render and display those pages and data entities to the user on a page visit. However, as the modern web evolves to include web apps and different type of app-like behavior more and more, it becomes more and more needful to serve the content to the apps and sites that do not run off your CMS or off your url. Like a weather app that queries a remote server but serves that data in your Android phone. So, in the decoupled model, an app or another website addresses your CMS and requests the data, and receives a response containing a dataset, which it then handles and displays as it sees fit.
2. Do you need a decoupled site?
So decoupling being cool and trendy, I see now decoupled blogs, decoupled busines card sites, decoupled everything. Built with React, their usual common traits are fast load times and very simple pages. However, you may not want to have this kind of a site yourself. Why?
Third, decoupled pages are usually very simple because given that cost of development, you don't want to pay for having additional page blocks, bells and whistles, unless you really need them.
If you decouple your site, you may end up with an expensive, SEO unfreindly, and functionally simplistic website. Do you want that? In some cases, you do, like, if you are having a news agency, a huge shop, or an online app. This is when you either don't care about SEO (an online weather app), or you have so much money that decoupling parts of the site will have sense financially, given the services you provide.
So, if you are having a blog or a web presence web site, that is not requested by the news providers, does not serve it's data to any apps, then there is no need to decouple the site for you. Even if you do serve data, providing a REST server and serving the data without the need to decouple the site would be the optimal approach.
In my observation, in most cases, it's much more efficient to use a hybrid approach and add app-like functionality to the site where it's needed, rather than to decouple the whole site.