One of the low reward strategies proposed by Stephan is to 'eat gently cooked food', as in boiling and gentle steaming over roasting, grilling, frying.
I have found this to be the case. For example take some potato, mash it with some fat and it's tasty to me but take that same potato, chop it into pieces and deep fry in the same fat and voila, high reward food that I am guaranteed to over-eat.
Same goes for the delicious charred coating of a steak or rib compared to their stewed counterparts.
Now, other reward properties make sense in an evolutionary context when not co-opted by food companies, they lead us toward calorific and nutritious goodness. Why would we be more likely to favour the taste of high heat cooking?
Caramelized sugars, changes to protein structure, the formation of unique compounds, etc. all create that meaty crust that we find so appealing. Food scientists are hard at work trying to uncover what exactly makes "browning compounds" so delicious (in no small part because they would like to figure out how to add these flavors to processed foods) but mysteries still about.
Some theories as to why we find these compounds pleasurable in the first place suggest that a nicely browned crust indicates cooked meat. The human gut was not initially a carnivorous one. Our shared ancestor with the chimpanzee likely has a more chimp-like diet of primarily fruit supplemented by a smaller portion of hunted game. When we moved to the savannah and began to rely on animal flesh for a greater portion of our nutrition, those who enjoyed cooked meat were able to consume/assimilate calories more efficiently. A survival advantage of only 1% is enough to increase a gene's frequency to 99% within a population after only a few thousand generations and we spend way more time than that as plain dwelling Pleistocene hunters.
Of course, this is just conjecture, but the fact that we do enjoy (and are "rewarded" by) a nice crusty piece of meat with a moist, juicy center indicates that there is some sort of survival advantage signalling at work.
Caramelization. This high-heat reaction causes the formation of many flavor compounds which = deliciousness.
Edit: As FED said we simply don't know. Browning is a series of very complex processes that we know little about. This is all just speculation therefore there is no answer.
For me it's definitely more of a textural affair; the crunch factor of roasted food and the contrast between the charred outside and softer inside is wonderful.
Benefits of low-reward be damned, I'm going to keep noshing on my roasted sweet potatoes.
Also, I've read that more intensely-cooked food has a higher caloric yield, though I'm suspicious of the notion that the difference is significant enough to induce such a strong preference for roasted food as compared to, say, steamed.