L-tryptophan is an essential amino acid. This means the body cannot synthesize it: it must be obtained from the diet. It’s been known for decades that L-tryptophan has niacin equivalent activity in the body (i.e., we can make NAD+ molecules from it). L-tryptophan is unique because it’s the only way to build NAD+ that doesn’t start from vitamin B3. It does this by a de novo synthesis pathway, which creates a niacin molecule through a series of biological reactions. So, L-tryptophan’s inclusion would seem to be a natural fit in a formulation that wants to support boosting NAD+.