No CDN means the origin is the public internet path. That is simple until the audience is not in the same region, or until a campaign and a flood arrive on the same afternoon.
Latency
Without a CDN, TTFB tracks distance to origin. With Cloud CDN, hits average 30 ms worldwide on the network. HTML has to be cacheable for that to apply to documents, not only assets.
Capacity
Origin pipes and workers scale linearly with unique work. Repeat views should not. 200+ Tbps and Origin Shield exist so launches do not require a heroic origin.
Security
A public origin IP is a public target. A distribution with restricted pull is not.
When you might wait
A strictly local intranet app with no public audience may not need a CDN. A public marketing site in one city still benefits from TLS, DDoS absorption and cache, but the geography win is smaller.