You get visibility into your usage, and you're seeing if you're exceeding the usage. They recommend to use plans if your typical traffic is 'only' up to 50TB per month. Occasional spikes are fine from what I understand.
This is not true. Even the Free plan has DDoS protection. L3/L4 (TCP SYN floods, UDP reflection attacks and similar) filtering is built-in and always-on, by default. CloudFront terminates TLS, and only forwards valid HTTP(S) requests to cache / origin.
The "Always-on DDoS Protection" on L7 is protection against massive requests spikes, built natively into CloudFront. Detection and mitigation of these attacks happens inline.
Does "data transfer" not mean CDN bandwidth here? Otherwise, that price seems two orders of magnitude less than I would expect