Differences between Cache and CDN usage

Assuming that the mentioned cache server does the same thing as a CDN, the main difference is the content distribution. In CDN it can be delivered by a server closer to the requester, which decreases the latency and therefore the user experience.

In some cases the content may be universal and "all" sites use the same URL to get, which helps to stay in the browser cache, so you may not even have to load jQuery that your site uses because it already is in the user's browser. This is only possible if you do not use a custom or hosted jQuery file.

These companies specialize in this, know how to assemble the infrastructure well, handle various attacks, you just have to configure to put the content there. It is usually highly available.

In compensation it tends to cost more in many cases.

The CDN has several mechanisms to help in the best performance. Caching is not always the most appropriate mechanism.

One of the advantages of this is that you can handle video streaming better than a cache that only works well with small objects. Even it can use multicasting which can greatly reduce bandwidth consumption and end up making the cost lower. Live content benefits from CDN for this.

Content that changes frequently may be better delivered by caching. But nothing prevents this change from being reflected in the CDN cache for other benefits. The function of your own cache is best used for this type of content.

It is not guaranteed that CDN will deliver better in all cases.