Why is a third degree burn, which destroys both epidermis and dermis considered a serious injury?
Third degree burns destroy all the layers of the skin including the dermis which includes the sensory nerves and the sweat glands.
Once the burn injury destroys the epidermis and dermis, the sweat glands will not be able to function properly which is to regulate temperature. The burned part would also cause severe and acute dehydration because of the massive loss of fluids from the burn.
The person would also not feel anything like texture or temperature. Skin grafting could help (ripping skin from your own buttock and sticking it to the burnt part) but it will not regenerate destroyed sensory nerve cells.