Why use CDN/caching?

Hi, even if you use Cloudflare, you can benefit from using Photon or Super Cache. There are zones in the world where the ping to cloudflare is unfortunately slow, i.e: any country in the south hemispher (Latin American countries, African countries, Australia…). Public in those latitudes may benefit by downloading smaller gzipped htmled pages by SuperCache, and dividing the bandwidth for images/css/js with Photon. Photon have a very good latency in zones where Cloudflare is slow. Cloudflare only help you save hosting bandwidth and is a layer of security against hackers, but this don’t mean that all your public enjoy a nice browsing experience.
It’s up to you. Try the tests at gtmetrix.com to see how is your site performing ??
Regards!

What about if I have a proper CDN, that CDNs images/css/js/everything and gzips? The only thing going from my domain would be the page itself.

That would be the same as cloudflare? I would then just consider whether cloudflare or my webhost is fastest for the single front page php?

Cloudflare caches images, css, javascript and places you behind their own IP addresses. They serve them from servers in the same continent as you. For example, if your host is in the US and you live in europe, cloudflare serves your images from a server in europe. I They protect you from DoS attacks.

Caching plugins serve a completely different purpose. They only decrease the load on your mysql server and php processing by serving cached copies on the processed php scripts.

Cloudflare does NOT cache html or php. However, it makes a snapshot of your website to show to users when your website is offline.