Half pads are available in various designs. They can be made of sheepskin, have gel padding or replaceable panels, be filled with air, or offer front or rear elevation.
Ultimately, they all serve to correct minor flaws in the fit of your saddle and to reduce the pressure on the horse's back by distributing it comfortably.
This helps you to adjust your saddle a little better, for example when your horse is in the growth phase or is being retrained after a break.