Pushing out new versions of your site. You can’t have the new assets on half the nodes that are serving your site otherwise your site goes down while things slowly propagate.
> Why is this the case? I don't have too much knowledge of CDN architecture so I am curious
Fastly is not really a regular CDN. It is a fully programmable edge cache with cache control algorithms decided and controlled by the customer running at the edges. You can think of Fastly configuration as a part of your code base where it is for you to decide if you want to perform the action on the edge on a per-request basis rather than on the origin per cached request basis.
That in turn means that if you do deploy to your API/web 50 times a day, you would are likely to deploy your Fastly configurations about the same number of times