What is the origin of the practice of making kids recite that "A sentence is a group of words which expresses a complete thought"?

Teaching rote recitation is one method of implanting that information in the head. If the kid later struggles with, "Is this a sentence," he might still be able to recite that years later.

I mean, I still do multiplication tables in my head when I need them. That's, what, third grade or so?

In the beginning there was the Tower of Babel. Then things got messy.