There is still a caching effect of the CDN for your servers, even if there isn't for the end user: if the CDN serves the file then your server does not have to.
Large CDNs with endpoints in multiple locations internationally also give the advantage of reducing latency: if your static content comes from the PoP closest to me (likely London, <20ms away where I'm currently sat, ~13 on FTTC at home⁰, ~10 at work) that could be quite a saving if your server is otherwise hundreds of ms away (~300ms for Tokyo, 150 for LA, 80 for New York). Unless you have caching set to be very aggressive dynamic content still needs to come from your server, but even then a high-tech CDN can² reduce the latency of the TCP connection handshake and¹ TLS handshake by reusing an already open connection between the CDN and the backing server(s) to pipeline new requests.
This may not be at all important for many well-designed sites, or sites where latency otherwise matters little enough that a few hundred ms a couple of times here or there isn't really going to particularly bother the user, but could be a significant benefit to many bad setups and even a few well-designed ones.
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[0] York. The real one. The best one. The one with history and culture. None of that “New” York rebranded New Amsterdam nonsense!
[1] if using HTTPS and you trust the CDN to re-encrypt, or HTTP and have the CDN add HTTPS, neither of which I wouldn't recommend as it is exactly an MitM situation, but both are often done
[2] assuming the CDN also manages your DNS for the whole site, or just a subdomain for the static resources, so the end user sees the benefit of the CDNs anycast DNS arrangement.
Large CDNs with endpoints in multiple locations internationally also give the advantage of reducing latency: if your static content comes from the PoP closest to me (likely London, <20ms away where I'm currently sat, ~13 on FTTC at home⁰, ~10 at work) that could be quite a saving if your server is otherwise hundreds of ms away (~300ms for Tokyo, 150 for LA, 80 for New York). Unless you have caching set to be very aggressive dynamic content still needs to come from your server, but even then a high-tech CDN can² reduce the latency of the TCP connection handshake and¹ TLS handshake by reusing an already open connection between the CDN and the backing server(s) to pipeline new requests.
This may not be at all important for many well-designed sites, or sites where latency otherwise matters little enough that a few hundred ms a couple of times here or there isn't really going to particularly bother the user, but could be a significant benefit to many bad setups and even a few well-designed ones.
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[0] York. The real one. The best one. The one with history and culture. None of that “New” York rebranded New Amsterdam nonsense!
[1] if using HTTPS and you trust the CDN to re-encrypt, or HTTP and have the CDN add HTTPS, neither of which I wouldn't recommend as it is exactly an MitM situation, but both are often done
[2] assuming the CDN also manages your DNS for the whole site, or just a subdomain for the static resources, so the end user sees the benefit of the CDNs anycast DNS arrangement.