As animators, our goal is to make the audience believe that the character is alive. For me, a big part of that is a convincing secondary action. Don't get me wrong - clear acting, good body mechanics, strong poses, etc. are all crucial and are all necessary in a good reel. However, once those things are there, it's those subtle human imperfections that take it to the next level.
Something as simple as breathing, blinking at the right moment, fidgeting, darting the eyes, if done right, can be so powerful by adding another layer of human believability to your character. These are just examples, of course, and it's important to remember not to pull the focus from your primary action. I like to think of secondary action as a "spice" that can be used carefully to add some extra interest to your shot or reel.
This is a big reason why I like video reference. If I only plan out in my head every motion that my character will make, I find it often results in an overly "choreographed" look. However, if I act out my shot in front of the camera, I often notice all kinds of little things my body does that I never would have consciously noticed.
In my opinion, the best reels are the ones that make me forget I'm watching cartoon characters and make me truly believe that I'm watching a living being.