To add, decoupling the frontend and backend can also be useful for client work. Clients love CMSs like WordPress because it's easy for them to mess with things, but sometimes they mess with things that they shouldn't. By decoupling things, they have the ability to make changes to content, but not to the frontend unless they know what they're doing.
Here's a good read that goes into why people would use a static frontend on a WordPress backend:
We're a place where coders share, stay up-to-date and grow their careers.
We strive for transparency and don't collect excess data.