Grammar lessons are a shortcut. So are books and other corpuses of knowledge. People have spent time documenting patterns that exist and its useful to learn from them instead having to brute force everything yourself.
This is assuming that the brain regions that learn rules overlap with the brain regions that develops fluency in a language. I think Krashen's hypothesis is that this is largely not the case. You can "fake" some degree of competence by learning the rules and using that brain region, but you're slow and not fluent until you expose the other region to enough real-world data.
