Comment by kmacdough

The biggest advantage? Regional caches.

If your traffic volume stays well below server capacity, the network connection is impeccable and throughput isn't a major factor in ISP costs, then the improvement is indeed negligible.

However, if the server or it's network ever experience congestion or disruption, or you simply face sheer volume, that's when CDNs shine.

Regional caches insulate clients from network or performance issues with your server or its connection. If you have high volume, they also drastically reduce traffic through the backbone which can reduce ISP costs, depending on your contract.

Like others say, this really applies to all static content, including static media, css and js.

I hear you ask, "how can you be exceedingly good at this?" Caching itself isn't tricky, but catching efficiently and effectively at such huge scales is. The fact that Cloudflare offers these services for FREE, is a pretty good (perhaps Faustian) deal.