What is the Appeal of Concept Books?

I ran across this article from the LA Times (warning, annoying ad before you get to the good stuff) about a new ABC book. It sounds truly neato, but one of the things that caught my eye was the gushy first paragraph.

Alphabet books are a secret passion among book lovers. The sheer number of ABC books published demonstrates that they hold a special place in the world of children's books.

Why are ABC books (and their brother, 123 books) so popular? Is it because parents who believe that the child needs to be learning something every second are buying them up like popcorn at a county fair? Or is it the child's delight in identifying letters and vocabulary?

Thoughts?