According to statistics, like [these](http://w3techs.com/technologies/overview/content_delivery/all) from W3Techs, a lot of websites let you load vital files through large third-party services like Google Hosted Libraries.
There are a couple of reasons why web developers are tempted to do this. It lowers upkeep costs (as these services do not cost the host any money), and it speeds up the web in the sense that if you store a specific version of a file once, you will only contact that central content delivery service to see if the file you have is identical to the one they are serving.
The fact that these companies are now deeply woven into the fabric of the web, and that telling them off actually breaks a significant percentage of all websites, has summoned this add-on. It comes bundled with a fair amount of commonly used files, and serves them locally whenever a site tries to fetch them from a delivery network.