Do parents actually think that grounding their children, exspecialy teenagers works?

I, as a teenager, nearly 18, don't think that grounding works. It makes me spiteful and has never changedmein the least. I recently read a resed question that asked if they thought grounding their children to his/her rooms when hsher grade fell below a C. This idea seems ridiculous to me because I know that I've taken cles to push me, to make me think, and i certainly knew I wouldn't be getting any sort of A in that cl. I did end up with D+ and i was happy because on paper it said that I hadn't learned squat but I knew, personally that I took more in that cl than any cl I have ever taken. My mom understood this, and she didn't ground me. And yes, she normally would ground me if I did have B- or worst. I us want to hera some reasoning behind this. Somehow it just makes more sense to me to give them a list of chores to doit with them while thye do thier homework, or at least to check on them than go online and only see the paper that shows nothing.