[Filligree]

How is it useless?

If the digest you provide is found, then you know the data hasn't been changed. Sure, that's the only bit of information you get, but in a lot of cases that's the only one you need.

It doesn't guarantee your data can't be changed, no. What guarantees that is your contact with Amazon.

[cloudhead]

Your other comment puts it nicely - "trust but verify" - I guess it's more about auditing yourself, than Amazon.