Because the unchanged module may depend on other modules which were just changed.

For example after refactoring and reinstallation of base class for call statements (I work in the telecommunications) it would be nice to check whether the derived classes (incoming call statements, outgoing call statements, ...) are still working.

Another real (my) world example is the upgrade of SOAP::Lite and XML::LibXML which we use extensively. I wish I checked our modules before this upgrade.