The concept of CDN (Content Delivery Network or content distribution network) is not clear to the most newbie. Even some blogger don’t know why do you need this? It is a large distributed system of servers deployed in multiple data centers in the Internet. CDN do deliver content partially or entirely of your website from their server instead of from your web hosting server which will make your site loading faster because the content is deliver from a server closer to the visitor. CDNs serve a large fraction of the Internet content today, including web objects (text, graphics, URLs and scripts), downloadable objects (media files, software, documents), applications (e-commerce, portals), live streaming media, on-demand streaming media, and social networks. In case of Blog user most of them want to host their .js or .css file to another CDN hosting site. Because it makes your blog loading faster because the content is deliver from a server that is close to the visitors.
Suppose your blog is hosted in US then your blog will load faster for those visitors who live is near USA. Fundamentally CDN has great effect to make your blog load faster for international visitors from around the world. By CDN you will have many mirrors of you blog all over in a global network. If your blog is made for global audience then you must consider CDN. However, if your blog’s target group is local audience with local content or product then maybe you don't need CDN.
Still you can consider to use CDN for caching, minifying, hotlinking guard, parallel request, and security. Some bloggers are using CDN but even then didn’t noticed that you may already using certain type of CDN in a minor scale.
For example if you have installed some bookmarking services that provided from addthis.com or sharethis.com, You won’t find any image file code from those but it load from respected bookmarking site that already deliver from global CDNs instead of directly from their server.
Even Pinterest providing their services from CDN server.
Some site do audio/video streaming for broadcast requires a lot of bandwidth. But you must be choose high performing network. But FreeCast broadcast your audio video stream with its P2P network. Even you can broadcast from your PC easily by installing the FreeCast software. This is a specialized Content Delivery Network solution. Peer-to-peer Processing (P2P) project was the famous Napster, and now bitTorrents, utorrents are using peer-to-peer file sharing protocol. So, as you may have guessed this type of processing requires you to have a software installed on the user’s computer.
I have just given an idea about 4 major CDN service providers among many alternatives. Remember that you may feel like there's no difference in the loading speed after using CDN (Content Delivery Network) services. But you won’t noticed about the effects of CDN. When the visitors from other location of the globe will feel the difference.