What does 'Copy Customizing for Project Management' do, and why must you copy the ChaRM/Requirements transaction types first?

It copies a preconfigured project type into a custom one and defines its task types.

You must copy the standard transaction types first because each task type has a one-to-one relationship with a ChaRM/Requirements transaction type. A PPM task type is only copied if its attached transaction type is already copied (or it has none); a project type is only copied if at least one task type needs copying.