If you're using Contentful then how is that different or better than wordpress or any hosted system that is made for writing posts?

The only 2 advantages of splitting this into separate systems is extra customization (even though hosted solutions are already very flexible) and compiling the entire site into static files. The 2nd seems to be more of a thing for devs to enjoy rather than something that makes a real difference to users, especially when you're already using a CDN.