When I think of automation, another similar word comes to mind, animation. The process of automating your track in my mind seems as though you are animating the songs. Making them more lively, giving your music layers, as well as creating a more dynamic, living creation. Sounds like I'm talking about Frankenstein's monster, but in reality every track that you mix and master is it's own, special creation. Automation helps you to individualize every session and add some flare. Things that can be altared using automation can include the volumes of each individual tracks in your session, panning, effects, plugins...essentially everything! It can be very simple things in a rock song, to very drastic and apparent things in an electronic song. It just depends on to what extend you want to utilize it.
Automation is extremely vast and can be used for many things, therefore I feel as though I still have much to learn on this subject, but I have used it in a few session already and that amount of color it can bring to a mix is amazing!