"Why do I have to learn grammar?"
1. Core knowledge. If you're educated, you know grammar. It's like learning state capitols. No one yet has asked me the capitol of North Dakota (it's Bismarck). But it's one of those things educated people learn.
2. Jargon. Grammar provides a vocabulary to communicate about writing. When your teacher says, "Great vivid adjectives," it helps to know what adjective means.
3. Be able to apply rules. One rule says, "Avoid passive voice; it makes dull writing." You can't avoid passive voice until you know what it is.
4. Understand writing better. Can you compose music without knowledge of music theory, major/minor chords, and dominant sevenths? Well, yes. But you'll write music better if you know music theory.
5. Analyze others' writing. A knowledge of grammar lets you analyze, describe, and eventually imitate the style of good writing.
6. Foreign Languages. Knowledge of grammar helps you learn foreign languages, especially inflected ones (all the European languages).
7. It's a mental discipline. The Army makes recruits do pushups. Now I've never yet seen a soldier do pushups when he's fighting the enemy. But building up those muscles equips him to perform his other tasks. So also, learning grammar strengthens logic skills, and you use logic elsewhere.
8. Equipping. I want to equip my child for whatever life calls him to. Even if today he wants to be a fireman, one day life might call him into a vocation that requires strong writing skills.