Docker is nice because you end up with the same configuration in development and production. There are many hidden details that "just let someone else host your site" or "just rsync your files to a server" gloss over. Who is renewing your TLS certificate? Where do you configure headers, redirects, mime type mappings, etc.? Where do access logs go? How do you update the version of the web server? What effects does that update have?
When you manually do these things, you rely on a bunch of implicit defaults. Maybe your production server and your workstation happen to have the same version of nginx, and happen to set the same defaults. So you can test a change on your workstation and the same change works in production. But more likely, that is not the case. So you get weird differences between development and production, and you only notice when you push to production. That is not ideal. Building an image with your webserver and static files ensures that you see the same things in both places. There is no need to test anything in production, as you have a copy of the exact code and data that is going to be running in production, locally. You can tweak and poke to your heart's content, confident that you'll have the same effect when you push to production. There is no need to maintain documentation about how to build your project and what versions of things you use; you specify them in a machine-readable format and the machine dutifully builds the project correctly every single time.
(One disadvantage of clean builds, though, is that sometimes you want old artifacts to exist. Consider a case where you use webpack to generate javascript. Typically, you'll output a bundle like "main.abc123.js" which is loaded from "index.html" via a script tag. What happens when the browser loads index.html from your last build, then you the next request goes to an updated server, which says to get "main.def456.js" instead? The page silently breaks, because the server doesn't have a file called "main.def456.js" anymore. "rsync --delete" has the same problem. And if you never delete anything, you eventually use an infinite amount of disk space. So there is definitely room for improvement here, but "it will probably work if I don't think about it and cross my fingers" is not the improvement we're looking for.)