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Hosting javascript from a CDN is useful for a few things

a) seperates static hostic (js/css/images) from dynamic hosting, which require different tuning and traditionally may run better on separate machines

b) you used to get some parellism benefit from multiple hostnames, that might be gone with http/2, but then you may not want head of lime blocking interacting between html and css/js

c) a good CDN will get your js/css/images to your users faster than if they got them directly from your origin servers, if your assets are of significant enough size to overcome the overhead of connecting to the CDN, and the chance that the CDN has a cache miss (depends on usage). A good CDN is using geo aware DNS and/or anycast to get clients to a datacenter that's near to them, and chances are your origins are only in one to three locations. For one site I worked on, when we did a redesign that doubled the page weight, but also moved serving everything behind a CDN, the page load time ended up about the same, so the CDN basically let us double our page weight for free (I would have preferred that we just keep the page smaller and get a faster site, but I don't get to make those decisions)

d) some people claim there's a reasonable chance of browser caching if you use a public CDN path, although personally, I expect that's very small -- there are so many public CDNs and so many versions of all the libraries and most browsers have fairly small local caches.