These are, effectively, different use cases. You want to use (and pay for) Express One Zone in situations in which you need the same object reused from multiple instances repeatedly, while it looks like this on-disk or in-memory cache is for when you may want the same file repeatedly used from the same instance.

Is it the same instance ? Rising wave (and similar tools )are designed to run in production on a lot of distributed compute nodes for processing data , serving/streaming queries and running control panes .

Even for any single query it will likely run on multiple nodes with distributed workers gathering and processing data from storage layer, that is whole idea behind MapReduce after all.

Also, aren't most people putting Cloudfront in front of S3 anyway?

For CDN use-cases yes, but not for DB storage-compute separation use-cases as described here.