If you don't, you manage your dependencies via import statements in QML files. qmlimportscanner hast to read those, and you have to manually run CMake everytime you add or remove a dependency. This is brittle.
The DEPENDS option states the dependencies of a module in a much clearer way and allows qmllint's and qmlcachegen's type inference to do a better job. In order to add or remove a dependency there, you edit the CMakeLists.txt file which automatically triggers a re-run of CMake.