为什么要运行“渐变干净构建”而不是“渐变构建”？

The clean task is defined by the java plugin and it simply removes the buildDir folder, thus cleaning everything including leftovers from previous builds which are no longer relevant. Not doing so may result in an unclean build which may be broken due to build artifacts produced by previous builds.

As an example assume that your build contains several tests that were failed and you decided that these are obsolete thus needs to be removed. Without cleaning the test results (using cleanTest task) or the build entirely (by running the clean task) you'll get stuck with the failed tests results which will cause your build to fail. Similar side effects can happen also with resources/classes removed from the sources but remained in the build folder that was not cleaned.

Other build tools like buck will detect that some tests are removed and won't run them without the needs to run clean target. I think this is pitfall of gradle.

It removes the build directory. (Build contains the output of the gradle operation)

You don't need to run the clean task. Gradle will track task dependencies and clean appropriate parts for you. Here's an example Gradle project I created to show that the accepted answer is incorrect.

If custom tasks don't track their dependencies well (they're bugged), then clean is a workaround.