So, I don't know much about Leatherworking, because I am very new to it, but I still wanted to be helpful. So, I used ChatGPT to at least try to answer your question.
You’re on the right track with your thinking! The leather pieces sewn under the fleece onto the leather of the skirting serve several purposes:
Support and Reinforcement: These leather pieces provide additional support and reinforcement to the skirting leather, which helps prevent stretching and distortion over time. This is particularly important in areas that experience significant stress and wear.
Protection: The extra leather layers help protect the skirting leather from the abrasive action of the horse's movement. By absorbing some of the friction and pressure, they help extend the lifespan of both the fleece and the skirting leather.
Shape Maintenance: They help maintain the shape and structure of the saddle skirt. The additional thickness and stiffness provided by these pieces help the skirt retain its form and fit better against the horse’s body.
Distribution of Pressure: The added layers help distribute the pressure more evenly across the horse’s back, enhancing the saddle's comfort and reducing the risk of pressure points that can cause soreness or injury to the horse.
Durability: By reinforcing the areas that are most prone to wear and tear, these leather pieces contribute to the overall durability and longevity of the saddle.
By understanding these purposes, you can better appreciate the importance of these components in the construction and functionality of the saddle. Good luck with your refleeing project!
Sorry if none of this helps.