The issue is incremental compilation. It is common (although not universal) that the way the build system (interpreted broadly) remembers that there are errors

The issue is incremental compilation. It is common (although not universal) that the way the build system (interpreted broadly) remembers that there are errors in unit X is that it has failed to produce a (new enough) unit X. In that setting, -Werror turns warnings from something that might not be surfaced in a (re)build into something that will be.