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CDNs are used to minimize latency which is why the second link (which is a link to the image copy on the CDN) is accessible, while the first link is not. In order to make the access as fast as possible, the access does away with any expensive privacy and security checks that the first direct link may require.


CDNs are also used to avoid having too many nodes access a specific node. It would be significantly more costly, in terms of bandwidth, to also host images and videos in the main servers, so CDNs are one way to offer specialization, where they only host images and videos and are strictly read-only.


Another thing to mention is in order to collect user data (click, keystroke), server tends to prevent client side caching by marking pages as "uncacheable". CDN is an intermediate way for both caching and data collection.


In addition to better performance (from minimizing latency), CDNs also save companies money by offloading the traffic served directly from their origin infrastructure.


Interestingly, Google only launched its own CDN very recently. The company made the decision to help app developers boost speed when hitting google's services.