Illness, sports, obesity are all valid reasons. I feel healthy eating can be trained like a muscle - tracking for a few weeks/months helps understand food choices better.

Too much sodium / low potassium are common in most diets. Low magnesium (sleep issues), low zinc, low fiber etc are a few others - a common remedy being nutrition supplements. Better tracking allowed me to eliminate additional supplements from my diet (except Vit D, which is hard to find in natural sources).

Famnom suggests FDA RDI defaults, but users are free to choose what they would like to track.