I think most of this has to do with school-age children. Generally, AFAIK, kids get a recess break around 10 or 10:30, and then lunch at 12, and nothing else until after school. Many kids where I came from would have to get up really early to catch a bus to school, and by early I mean ON the bus at 7am or earlier to be there for 9am. It’s just unreasonable to expect a child that isn’t hungry at 6 or 6:30am to be able to last until recess time to eat their first food of the day. This is where the “most important meal” idea comes from, IMHO. A hungry child cannot concentrate as well as one who isn’t hungry, and this DOES reflect in the childs work (IANATeacher, but my mother has been for 25+ years and I have spent a lot of time talking with her and helping her out over the years). Many schools now have a free breakfast program, or something for a dollar a day, in order to be sure that hungry children are fed in the morning. Originally, this was only for underprividleged children, or those coming from households where noone made efforts to feed them in the morning, but increasingly, at least at my mom’s school, the program is being expanded to serve ANY hungry child, because even if they had a bowl of cereal at 6am, odds are they are hungry at 10.
Of course, the quality of the food is equally important, and I would think would have a HUGE bearing on how important the meal actually is, but in some cases, children in particular, getting ANY food into them is more important than letting them starve. And as I said, it DOES reflect in their grades, hence the added push for the “importance” of breakfast foods aimed at children.