1) for reproducible builds that always give the same result whether or not qt5 is installed
2) in situations where the compilation of qt5 related code results in a build failure, so rather than to spend hours debugging the issue, the user can go on and disable building of the unwanted gui
3) for profit - save build time and disk space
1) for reproducible builds that always give the same result whether or not qt5 is installed 2) in situations where the compilation of qt5 related code results in a build failure, so rather than to spend hours debugging the issue, the user can go on and disable building of the unwanted gui 3) for profit - save build time and disk space