Make Clean Vs Make Clobber

make is pretty smart, and picks up what has changed from the last build, so if you run repo sync and then build without cleaning, in most cases it should work just fine.

One exception would be after adding or updating proprietary vendor binaries. Per Google’s Android documentation, Building for devices, Google does recommend:

Cleaning up when adding proprietary binaries

In order to make sure that the newly installed binaries are properly taken into account after being extracted, the existing output of any previous build needs to be deleted with make clobber.

make clean does the same thing as make clobber, which is removing all generated files including those in ($OUT_DIR) if defined.