It's not just "guarantee levels of service", it's permission to sling lots of bits (and of any "type" you like) without being deliberately cut off. It's not like you're just paying for more 9s at other CDNs (or at CloudFlare).

Yes, the free tier is in fact pretty amazing if you have mid-traffic-or-lower site and are mostly serving HTML pages and basic images, js, and css for those pages, and the stakes for availability are pretty low. Which describes quite a bit of the Web.