The Lantus at night is to keep your blood sugar more normal all day long inbetween meals. This takes care of the glucose the liver makes when food is not available. This is called a basal insulin.

The bolus at meals with fast acting insulin like Humalog is for what is needed when a person eats...When the pancreas is totally burned out, no bolus with meals occurs. So the Humalog replaces that. When a person follows strict diets, the meal does not require a real spike of insulin...the diet helps control this. This is why the proper snacks are important, and small meals more often. Things like nuts, cheese, protein, etc tend to not spike insulin like crackers or cookies do. Also slowing the stomach emptying can help reduce insulin spikes (so nuts again and other good fats).

But in uncontrolled eating or large meals, or lots of carbs, a bolus of insulin has to come from somewhere.