I agree with Crowndragon’s first post, the horses are rested afterwards which is what they needed to heal anyways.
The “idea” is that the healing process from pin firing and blistering brings increased blood flow to the area to help in the healing. Not sure if I believe it. Magnets are supposed to do the same thing and I don’t believe that either.
Long term I don’t think it causes any issues, but my whole bucket of experience is from a total of one…a standardbred named Caramel Chewey who was apparently pinfired for a curb injury.