because little kids dont understand health as a concept they need an actual measurable goal. thats why we were told that eating crusts makes your hair curly, carrots make you see in the dark and veggies make you taller.

Actually, you're missing the point of how parents use that phrase. It gets trotted out when kids are scarfing down an unbalanced meal, going right for the mac&cheese or the sweets and entirely ignoring the veggies. A diet that lacks vital nutrients, be it strictly herbivorous, strictly carnivorous, or omnivorous, will not produce a strong and healthy body.

you know that an omnivorous diet is what we have currently and it means a diet consisting of everything (prefix omni) therefore an omnivorous diet done correctly is healthy. if your comment was pointing at me then i know why parents use it but i was stating why it works on kids.