That's the reason why it's important to advertise children as early as possible. Because once they decided on a product it's very hard to convince them of something better.
And that segmentation ensures that it isn't just a once a lifetime choice. Each to time I look at the Colgate shelf I have to consider what is more important to me this week - the state of my gums or the likelihood of cavities.
Grocery stores also like to offer their customers the illusion of choice, while dealing with as few distributors as possible. This simplifies their logistics. They would prefer to sell ten varieties of Colgate and Crest than ten separate brands of toothpaste. It gets ridiculous when you see half an aisle of Wheat Thins (original, whole wheat, low salt, spicy, plus the store clone brand of each of those). But I suppose the nice consequence is that small corner markets can have just as good a selection, effectively, as huge superstores.
I take the same approach with many boring shopping choices.