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Assume you build a package like beginner_tutorial. This depends on package roscpp.

This means roscpp is the first package, beginner_tutorial the second.

Now somebody else wants to create another package, additional_stuff, on top of beginner_tutorial. That would be a third package in our dependency chain.

If additional_stuff would also need roscpp to depend on beginner_tutorial, then beginner_tutorial should declare this, so that additional_stuff does not need to declare it as well.

This is not just about run-time, it is about build time of the 3rd package.

E.g. if in beginner_tutorial you create a public c++ header that includes stuff from ros.h (from roscpp), then additional_stuff would need to know where to get ros.h. So this is not exactly run time.