Content Delivery Network (CDN)

A Content Delivery Network (CDN) means that your site is cached at various places worldwide, so that static parts of your site are always available close to a visitor.

This surely sounds good, but I doubt if in reality, CDNs work at all.

When surely not to use a CDN: When your customers are all located in the same geographic area (e.g., an Hungarian webshop). In such a case, better just host your site in that region.