That slogan is an oversimplification but if I eat a bowl of steel cut at 7am I won't snack or be hungry until 2pm or later. Whether or not I eat this will have a strong impact on my day. Sounds important IMO
Depends on the person. If I eat breakfast I feel like crap for 2-3 hours afterwards. I have friends who can't function for the day before eating first.
It's certainly not the most important meal of the day in general. Especially as a societal meme that came directly from an advertising campaign for a cereal manufacturer.
— Advertising slogan, Kellogg’s 1917