The fat that comes round the edge of a piece of meat is fairly solid, no way could,you spread it. Yet if you put that piece of fat into the oven and cook it slowly, it renders down into the stuff that we know as dripping, leaving behind an inedible, shrivelled bit of indigestible stuff that was originally the piece of fat. ( I do this with the goose fat at Christmas, to get enough goose grease to cook spuds in through the year)
Therefore I can only conclude that heating the fat renders down the good bits, leaving behind the bits that make it undigestible in the first instance.