You can receive form submissions, which is very nice for some websites. You can also bundle assets and minify CSS, JS, etc. You can set header rules and cache settings. All of this is not possible with Github as far as I know.
idk if GH pages has added this, but Netlify lets you determine your build command and environment for deployments. You can also have different deployments per branch with easy rollback functionality. In short, its built for this exact problem where GH pages is a tacked on feature for repos.
You need "different deployments per branch with easy rollback functionality" to run a blog with a couple markdown files? Talk about over-engineering.
There’s utility for different deployments per branch and rollbacks when you’re connected to a headless CMS (Contentful, Wordpress API) and managing more complex statically-generated web sites.