Once you upload your site to your web hosting server it gets cached on other servers around the world that are part of the CDN. For example, if you serve your website content and images from a hosting company in the UK, the ping(delay) to users in Asia will be more than 400ms because of the distance. If you upload it to a CDN though, when the user requests the content he will be pointed to a server in Asia closer to his location and the ping will be tens of times lower.
The benefit of the Content Delivery Network is mostly speed. This can improve your user experience and SEO. Also, when you deliver the content from different servers it is much less likely to have a crash on a server due to heavy traffic since the users from different locations request the content from different servers.
