I think oftentimes the benefit of a CDN is not (just) performance, but also cost--sure, nginx can serve your pages plenty fast enough, but how much is your host

I think oftentimes the benefit of a CDN is not (just) performance, but also cost--sure, nginx can serve your pages plenty fast enough, but how much is your hosting provider charging you for that network traffic? The answer is generally "more than a CDN would be" (once you've used up your free bandwidth for the month, of course)