(I assume) it will fail if the eTag doesn't match -- the instance it got the header.

The main point of it is: I have an object that I want to mutate. I think I have the latest version in memory. So I update in memory and upload it to S3 with the eTag of the version I have and tell it to only commit if that is the latest version. If it "fails", I re-download the object, re-apply the mutation, and try again.