it can be useful for some coordination problems.
for example imagine that you have N writers and you want only the first of them to write something in a n object.
each writer writes their content into "_foo" and each tries to copy that to "foo", with "x-amz-copy-source-if-match". Only one of them will succeed and "foo" will have one consistent value, which all observers (including other writers) can agree who has won.
