On QTIFW Manual Operations section I read

The operations are prepared by component and controller scripts or component.xml and performed by the installer.

I can see why operations can be performed by component and controller scripts. But why/when should one perform operations from component.xml? Seems the same as performing operations from components, isn't? Should I understand this as two ways of doing the same thing?

I understand component.xml is supposed to be what's called package.xml on Package Directory, isn't? Two names for he same thing?