One of the main ways for a content delivery network to accelerate a piece of content is to cache it on an edge server in close proximity to the visitor. Once cached (usually after the first request) the content is served to all visitors at high speed, without the need to contact the origin, which may be physically located in a distant country or even on another continent. This greatly accelerates the rate at which visitors can fetch a copy of your content and it also allows you to scale seamlessly since all the load from the origin is shifted to the CDN.
However, there is also a down side to caching. Every now and then you may wish to update the contents of a file, or simply push a new version of a software without changing it's file name. Since the old version will still be in the CDN cache, the new version will not be served until the cache is considered expired, unless you let the CDN know that it should fetch a fresh copy.