Q1. What’s the difference between focused and selected?
As you have seen, when performing an edit (“paste”) it is the selected track that is altered. The same is true when applying effects.
That’s the difference.
Q2. Why is track focus necessary at all?
Imagine that you are unable to use a mouse, touchpad or other “pointing device” and can only operate Audacity via a keyboard (as would be the case if you were blind, or possibly had severe arthritis). “Focus” allows you choose which tracks to select or un-select.