Content life cycle project and children channels


I have a misunderstanding regarding the channels that are available to be attached to a project.
First, I select a base channel for the project, then, on the child drop-down menu, you are allowed to select also other base channels (but referenced as children?).
I am not sure why is it possible as I thought a project should refer only to one base channel and its possible children channels.

Does it make sense to have a project with multiple base channel and so potentially different OS versions?
My possible answer is that you may want to add some custom channels with customs packages for the organization?

Many thanks!

Some customers want to create separate trees for there custom channels. This feature allows to mix them together.
The customer is responsible to not mix impossible combinations.