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Why Use a Public CDN?
Performance

Content delivery networks, in general, can reduce your page response times because they are able serve resources from multiple geographic locations, lowering the distance the data has to travel to reach the user. Compared to multipurpose web servers, like the ones used by shared web hosting providers, CDNs also have optimized backend technology specifically for sending out static page resources such as images, JS files, stylesheets, and so forth.
Ease of use and maintenance

It’s a hassle to upload and update open source libraries on your server. With free public CDNs, all you have to do is reference your dependencies and they will just work. And, if you were away on vacation or aren’t keeping up with project updates as much as you should, you have little to worry about because the public CDN will likely update the files they’re serving.

Reliability

The free public CDNs in this list are backed by major tech companies, giving you a bit of assurance with regards to their trustworthiness and service performance.
Higher cached probability

Because public CDNs are used by many websites, there’s a bigger chance that the user coming to your site already has a resource stored in their browser, further improving your page response times.
It’s free

In my definition of what a “public CDN” is, the service should have no fees and should be accessible by anyone, just like Gmail or Dropbox.