It helps you learn what is expected at shows, how the classes work, what the judge is looking for without going to the expense of a regular show. One wears proper attire but the atmosphere is pretty relaxed.
It is a more relaxed show not just for beginners, it is kind of a warmup for larger shows. Test to see hwere you are at so far, what you need to improve on, If your horse is new to shows this is a great way to get him used to the atmosphere of one. Often times the judge after a class will speak to you and let you know what you need to work on. Give tips. You can recieve help by your trainer from the rail through your class often times. But you still do put on the performance like normal show I.e. Full show attire, well groomed braided horse, clean tack. When I did show every season I would enter in at least 2 schooling shows before I went to my larger shows so I would know what needed to improve on. Also gave me a good idea about my competition for that year. I did this as a beginner and as a well seasoned rider. They are very helpful, and much cheaper.