Maggie's Farm

I have written professionally for over 50 years and have not once found a use for diagramming sentences.

Writing has value only as a tool for communication, and communication for specific purposes. Your son's advert is similar to the "telegraphese" of an earlier day (price per word), or how modern radio operators use a common shorthand (Q codes or abbreviations like XYL or 73). Regardless, the human mind understands words and grammar even when mangled -- you'd have to ask a linguist why.

What is the point of knowing how to diagram a nongrammatical sentence whose meaning is perfectly clear? e.g., "batter up!" or diagramming powerful rhetorical devices that defy grammar such as epizeuxis, e.g., "Reputation, reputation, reputation!" Othello, 2, 3.

On every associate's desk in my firm is a copy of Strunk & White's Elements of Style. So they are at least competent writers in modern business and legal English. As an aside, words like "hereto" are banned. I doubt any of them could diagram a sentence, and they all communicate perfectly well in the shorthand text on their smartphones.

The proof of competency in writing is did the judge grant your motion, did the customer sign the contract, did your story get published. It is not whether one can diagram a sentence.

If you want to polish writing beyond the competent stage, get a copy of Farnsworth's Classical English Rhetoric.