Watch dancers 100% of the time (connect with them)
Catch and correct mistakes more consistently
Adjust level of difficulty instantly
Custom tailor workshop for problem calls and/or concepts
Resolve quickly when squares break down
Develop a mastery of choreography
I've already seen many of these benefits in my very first club calling experiences.
Preparation
Things tend to go well when I prepare, and not so well when I don't. Areas I've prepared go much better than areas I haven't. If I work on one area to exclusion of others, that one area is usually the only area I feel good about after I've called.