Position – When to separate dimensions?
If you animate along a path it most often makes sense to keep dimensions together, as you can easily edit it and easing keyframes will affect the overall speed of your animation along the path.
On the other hand if you want to animate things where the speed in the x and y direction is treated differently, for example bouncy movement, a wavy motion or if you only want to animate just x or y it makes a lot of sense to separate the dimensions before starting to animate.
Imported File Layer vs. Composition vs. Form Layer
When to use what and how to decide?
Also watch this great tutorial about the little star icon
Position vs. Anchor Points
Difference between Position and Anchor Point and how to move the Anchor Point.
Speed graph vs. Value Graph