See also the https://reproducible-builds.org/ website
build-compare is a tool used in the Open Build Service (OBS) of openSUSE to check if a newly built package is different from the old one. If it finds changes, dependent packages are rebuilt, the new package(s) are published and synchronized to the download mirrors.
However some packages contain build timestamps and other non-determinism and comparing these with other builds of the identical software will always return a change, which is wasteful on the build-power we have.
Apart from that, it can also increase trust in the openSUSE binaries if everyone is able to produce the same binaries on their own from the published sources (when using the same versions of build-time dependencies).