Concrete example: I have a deployment skill that knows my exact stack (Laravel on Forge, Postgres on a specific host, Cloudflare DNS). Without it, the agent spends 2-3 turns asking about infrastructure details and guessing configs. With the skill file, it gets it right first try.

Another one: a Chrome extension build skill that documents a quirk where content.css needs manual copying to dist/ after Vite builds. Without the skill, the agent hits that error, debugs for 5 minutes, discovers the fix. Next session it forgets and does it again. The skill is 3 lines and saves that cycle every time.

The key insight: skills aren't about teaching the model to code. It already knows how. They're about encoding the stuff that's specific to YOUR project that the model can't infer from context. Think of them as persistent post-it notes, not tutorials.