The point of CDNs is to get your content cached all over the region/globe as close to your customers as you can with sufficient capacity to survive DDOS or even just surge traffic from going viral. All but the biggest companies don't want to invest in that much hardware and software capacity to stage servers in that many different data centers for only occasional peak usage needs, so they buy CDN services from someone who specializes in it.

Unfortunately, since it's a pretty expensive business to get into, it tends to settle into something of a natural oligopoly of only a handful of big companies.

@orbitalmartian
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