Comment by viraptor

It depends on what you're trying to serve, but it's used to either save you money or as an insurance (or both). You don't need a CDN overall. But if you grow large enough, at some point you'll run into one of these three situations:

- Your public traffic costs you so much to repeatedly process that it's cheaper to let some service cache the common responses instead. (Where the cache size is larger than anything you'd want to support yourself. For example, if it's <1G and survives your service restarts, you may want to do it yourself)

- Your customers on the other side of the world start complaining that the resources take ages to load.

- Someone floods you with enough traffic that you can't respond to real customers traffic anymore. You get a ransom email to pay them to stop. But there are enough groups doing that that paying is useless because someone else will try again in a few days. If you're providing a service where people pay you to use the website, you're losing money until you solve this problem.