See www.thehorseshoof.com for an excellent image of the circumflex artery.
More images of the vascular supply to the foot on The Hoof Blog.
Nearly all the arterial (i.e. carrying oxygen and nutrients) blood supply to the solar corium (that the sole that we see grows from) comes from the circumflex artery. The solar corium is sandwiched between the sole (and therefore ground) and the bottom of P3, and can easily be damaged by compressive forces. If the horse is forced to stand or walk on the sole beneath P3, the sharp bottom edge of the bone can reduce or cut off the blood circulation to the solar corium, which can cause necrosis of the sole.
See: Clinical anatomy and physiology of the normal equine foot - C. C. Pollitt 1992
According to Pete Ramey in Boots and Pads: A True Breakthrough in Healing, a thick calloused sole can help support a laminitic horse temporarily during rehab while the walls that are less than perfectly attached are removed from active ground pressure. However where the sole has been rasped, or damaged by acute or chronic laminitis, it may be too thin (or sensitive), causing pressure to reduce or cut off the blood supply from the circumflex artery and thereby reduce sole growth. Using boots with thick pads usually makes the horse comfortable enough for movement and the pressure and release that will build new sole (once a realigning trim is in place and the horse is off all pain relief). Not protecting the sole can lead to bruising of the solar corium and abscessing.
Following rotation (and distal descent), the tip of P3 starts to compress the solar corium. If the rotation isn't corrected with a realigning trim (to return the palmar angle to around 5 degrees, return breakover to a point in line with correct new growth from the coronet, and reduce separating forces on the walls), the pressure of the bone on the sole is likely to increase, potentially leading to bone loss (osteolysis) of P3 and necrosis of the solar corium. This in turn can lead to abcessing, penetration of P3 through the sole, or incurable sepsis. This is likely to be one of the major reasons for the euthanasia of horses with laminitis.
In his excellent book, The Care and Rehabiliation of the Equine Foot, on p 351 Pete Ramey says "At the first signs of laminitis, restore P3 to a more natural ground plane, relieve pressure on the walls, and pad the sole with foam rubber - vertical sinking and destructive pressure to the solar corium can be prevented." He says that he has used these methods on over a thousand horses with laminitis, and has never seen sepsis develop. "Immediate action to eliminate constant pressure on the solar corium and separational forces on the laminae and the coronary papillae can be more important that anything else you do."