Rice with dal, roti with chana, khichdi combining both: the cereal-legume pairing runs through Indian eating everywhere. The reason it recurs is a straightforward fact about protein composition.
Protein quality is about composition, not quantity
Dietary protein is a source of amino acids, and several of those cannot be made by the body and must come from food. A protein source is judged by whether it supplies all of them in usable proportion.
Falling short in one of them limits how much of the rest can be used for building tissue, regardless of how much total protein was eaten. That constraint is what makes composition matter.
Most plant proteins are complete but skewed, carrying plenty of some essential amino acids and comparatively little of one or two others.
Cereals and legumes are short in opposite places
Grains such as rice and wheat are relatively low in lysine while carrying sulfur-containing amino acids adequately. Legumes are the reverse, rich in lysine and comparatively short on the sulfur-containing group.
Eaten together, each covers the other's shortfall, and the combined mixture is far better balanced than either component alone. The pairing works arithmetically.
The timing requirement is looser than once believed
Older nutrition writing insisted that complementary proteins had to be eaten in the same sitting. Current guidance is that the body maintains a pool of amino acids across the day, so exact meal timing is not critical.
That said, eating them together is simply how the food is served, so the question is largely academic for anyone eating this way. Practice arrived at the answer before the explanation did.
The pairing is not unique to India
Beans with corn tortillas, chickpeas with flatbread, lentils with barley and soy with rice all appear across unrelated food cultures. Populations relying on plant staples converged on the same solution repeatedly.
The convergence is evidence that the pairing was arrived at through outcomes rather than theory. Diets that worked were kept, and the biochemical reason was described much later.
What it means for cooking in the US
An American cook adapting Indian vegetarian food should keep the grain, not treat it as filler to be cut. Serving dal alone as a soup removes half of the pairing.
Anyone with specific dietary requirements should take individual protein questions to a dietitian rather than to a recipe. The general pattern is well established; individual needs are not something a cuisine can answer.