Wanna learn to make videos that use documents
When you want something to happen on screen, at the very minimum, you need to tell it:


When to start
When to stop
Where to be


Motion lets you create templates for generators, transitions, and titles.  So you'll build the details of an effect in Motion, and then apply, and re-apply that template in FCPX as often as you need.  You can also decide which parameters you'd like to retain control over while designing your templates in Motion, and make them available to Final Cut, so that you can tweak elements as necessary. 

Designing the movement of these graphic elements is the domain of a profession known as "Motion Graphics."  Like any other artistic practice, being a good motion graphics designer requires much talent, dedication, and practice.

Often, motion graphics designers will sell their designs, so that people who are less practiced can apply a polished look to their own projects, the same way a typographer might design a font for licensing to a broader audience.  If you search the web for "Apple Motion templates," you will find many of these pre-designed packages, complete with built-in controls for tweaking various elements to taste.