Your mood ratings create a record of how you're feeling over time. It's very useful to be able to go back and look at previous events and your reactions to them.
A core part of Cognitive Behavioral Therapy is being able to understand how your thoughts, feelings, and behaviors affect one another and your physical well-being. Each part of the cycle has the ability to affect every other part. Being able to look back at your mood over time helps you understand how you reacted to things in the past in order to better prepare you for them in the future.
You can also click on the green bar below the graph in Your Progress in order to cycle through your health and activity data to see how it relates to your mood. In this way, Pacifica lets you understand the parts of the cycle that relates your behaviors and your feelings. You can always click on the existing mood ratings to see your notes about what was going on at a particular time as well.