Cooking has more than one advantageous , even to the cave man.

1) Raw food, especially meat, carries parasites and organisms that can make you ill. Cooking easily kills these organisms.

Over decades, humans must have observed that people eating hot food are sick less often than people eating uncooked food, hence making this a social custom

2) Cooking makes the food tender. The proteins in food (especially meat) break down and makes it easier to eat. 

3) In addition, the fire at the hut always kept the wild animals away, which was an added advantage. 

All these factors made cooking very popular. Possibly, (though I have no evidence of it, and I even doubt it) humans have evolved a little to adjust to cooked food. This may not be true if cooking grew only 40K years ago. Other estimates say that cooking existed even before homo-sapiens (millions of years ago)

In this case, we have probably evolved our bodies, and digestive systems to the different ( and less taxing) demands of cooked food.