Many people get custom saddles so it fits their horses perfectly. You do not want to buy a saddle and have it pinching on your horses withers or hitting their back and causing them to be back soar. This is similar to what Rebecca is saying about the saddle putting too much pressure on her horse's back because it did not fit correctly. You want your horse to be comfortable so they can perform their best. For example, having a saddle that is pinching your horse is not good for jumping because it is limiting their movement/making it more difficult and uncomfortable for them. Custom saddles are common with horses who have a strange fit which can be because of high withers, etc. They are also expensive so you want to get what you pay for! You as well need to be comfortable and you do not want to have a saddle that is pushing your leg too far forward or backwards, etc. Hope this helps!
If you are going to spend money on a saddle you want it to fit you and your horse perfectly.
It fits you and your horse, it will be unique and it will bring more comfort for the both of you
My horse had problems with the pressure my old saddle put on his back, the tree was too wide so the saddle fell down and pressed on his withers, causing bumps that eventually scabbed off and left ugly white scars, I got a custom made Devoucoux saddle that fit him perfectly and was very comfortable for me, it cost me $5,000 but was definitely worth it because it stopped his bumps from forming and he is much happier in it
A custom saddle is a product that is quite complex. In my opinion, a saddle "made to measure" really does not exist, because the morphology of the horse evolves. It is like us, weight gain or loss, muscular change, growth ... But we call it a saddle because the saddle is originally made according to the measurements of the horse and the rider. A custom saddle can also allow a sensitive horse to no longer have back pain.
On a custom saddle, you can modify (among other things) the padding, the opening of the withers (for the horse), the seat and the quarters (for the rider). After that it has some stroke too !! I would say it's about 3500 € ... :)