Lettuce has 5 calories (41 kj) per cup (36g). For that same portion, celery has 14 (58.8 kj). Eggplant; 25 (105 kj). Considering every human needs ~1,800 calories (7560 kj) daily to survive and food used to be harder to come by the pre-modern world (or at least required a lot more energy to harvest), why did societies bother putting energy into raising these crops at all to the point that we still have them today? (though at least today in rich nations that struggle with obesity rates, they offer options for weight loss diets). Or did they just enjoy the taste and allocated some energy into these seemingly frivolous crops? Because it would take a f***ton (scientific measurement term) of lettuce to keep one person alive for one day. Thank you!
Is there a particular time period or location that you're interested in? As it is this question literally covers any conceivable time in human history, so you're not likely to get an answer that can deal with the issue in detail. This question is fundamentally about food cultures and why people grow what they do, and that will depend on different conditions around the world.