i had to do sentence diagramming in 7th and 8th grade. the only people i know (around my age, 20 or so) who’ve diagrammed sentences have gone to a catholic school (like me), except for one.
and you know what? i’m a writing tutor at my university, and the people who come in and have had horrid grammar have never diagrammed sentences. the ones who have excellent grammar (and just need help with essay structure or whatnot) have diagrammed before.
i’m not saying that the only way you’ll be good with grammar (and ‘good with grammar’ is most likely grammatically incorrect :D) is to diagram sentences, or you need to do them to be good at grammar, but in my anecdotal experience, it seems to help quite a bit. i can’t figure out why exactly it helps, but it does.
i still have my notebook from 8th grade (i saved it for the diagrams!) and you should see the structure. sure, “mother baked mary a cake” looks nice and simple, but “to win the race was her goal, but she sprained her ankle badly” looks like a 3D game of pickup sticks. if anyone is interested, i can scan some and post links to the images to see what they look like.