Poly pads have no medicinal properties as far as i am aware, other than being nice and cushy....
I use them, because I always have.. having have TB's the standard numnahs have a habit of rubbing, so I went to polypads, which i could position where i want..
If you plan to use a polypad, please tell your saddler so they can fit it WITH the polypad underneath. I have a WOW saddle and had to increase it by 2 headplate widths to use my polypads! If you put a polypad under a saddle that was fitted without it will make it pinch the horse and will make him v uncomfortable!
Can do more harm than good if user is not careful. need to pull them well up into the gullet of the saddle, because they're so thick with no tabs to attach them to the saddle they slide back down.
The saddle is designed to take the weight off the horses spine but the poly pad often sits flat across the spine and then the saddle sits on top so the weight is then directly on the horses spine.
Like most things they need to be used with care but often aren't, can't help wincing when I see horses ridden in them because they are rarely pulled up nicely.