The golf club only stays on the target line for a short amount of time. It does however need to enter and exit in a manner that will spit the ball out straight and eliminate curvature. Here is a visual to how it works.
Footwork is about stability but also motion. Watch this video to learn how one of my students created a step drill to help his footwork and body move in unison throughout the golf swing.
Swing plane is not as mysterious as many believe it to be. Being great at the exit plane will help clear up and downswing difficulties. It's better to know where you are going than where you are going or even coming from.
I get many comments about a video I made several years ago named "Out Is Down". Here is another example of it -with Patrick Reed- showing how hand path and club path are different.
Lag of the clubhead is an important benefit to releasing the club late in the swing and with power. Drill 4 is one of the training protocols in allowing our body to do such a thing.