actionlib_msgs required as build and exec dependencie, but not mentioned in tutorial.

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I am trying to implement my first action server on my own. I am following the following tutorial closely:

https://wiki.ros.org/actionlib_tutori...

After modifying my CMakeLists.txt as described in the tutorial and adding

<exec_depend>message_generation</exec_depend>

to my package.xml, catkin_make demanded actionlib_msgs to be a build and an exec dependency. So I added

<build_depend>actionlib_msgs</build_depend>
<exec_depend>actionlib_msgs</exec_depend>

to the package.xml.

My question is why this is required and why it is not mentioned in the tutorial.

I am using Ubuntu 20.04 an Ros Noetic.