What can a mechanical horse teach me that my real horse can't?
The simulator can teach you the correct position in the saddle whether you are working on balance issues, equal weight in your seat bones, half halts, leg position, transitions, leg yield, shoulder in, flying changes, tempi changes etc without getting fussy or tired. You can do the particular movement over and over until you get it in your body and gain new muscle memory. Most horses would not tolerate the same movement being tried over and over and over.
The simulator does not rear, spook, buck or bolt so you can concentrate solely on yourself and not worry about what the horse is doing. How much of your ride with your horse can you concentrate solely on your position?