I'm not entirely sure why it's done, I've only seen people do it when makeing panceta in Dalmatia. I've seen people simply put two appropriately sized pieces of plank on top and below and tie it firmly with pieces of rope to make and improvised press, and then some people have custom made wooden frames where meat is placed and then covered with wooden lid and pressed down with clamps. For how long or why exactly it's done, I don't know, but when we make panceta it is thoroughly dried to the point of being like jerky, so you can eat is just like that without cooking it, so my guess is it's to drain away as much fluids as possible.