Sometimes I'll read a book

Sometimes I’ll read a book to my daughter which will show a large, usually uppercase, letter next to a picture or ideogram, such as an apple or a cat. So, an “B” will be next to a bird and an “Z” will be next to a “zebra.” Nothing interesting. But when one “reads” the book, one says “B is for bird, C is for cat, D is for dog, E is for Elephant” and so on. Why is the letter “for” something? How did it originate that a letter is “for” a word? Shouldn’t it be “Bird starts with B, Cat starts with C, Dog starts with D, Elephant starts with E” — or is this just non-alliterative and unfriendly. I think a lot of our language works this way, whether it’s childhood-related or not; a series of words stick and they are used to make sense of the world regardless of their internal logic. I just wish I was more conscious of this in daily, speaking life.