What is the task of catkin_package() ?

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I have made many researches about this CMake macro, but I cannot understand what the task of this macro is. I have also read CMakeLists.txt tutorial in the link http://wiki.ros.org/catkin/CMakeLists.... In this tutorial, the purpose of this macro is not clearly specified, I think.

The most informative page for this macro is this link: https://docs.ros.org/api/catkin/html/...

However, this is not sufficient, I think. For example, I could not understand why we do not write all packages in find_package() macro into CATKIN_DEPENDS section of this macro.