CDNs cache static content at edge locations worldwide. User in Tokyo gets the file from Tokyo, not your US server.
For media-heavy systems (YouTube, Netflix, Instagram), CDNs are required. Latency drops from ms to ms. Origin server load decreases dramatically.
In system design: "User uploads go to S3. We serve them through CloudFront (CDN). Cache TTL of hours. Invalidate on update." This pattern appears in almost every media system.