The "Rules", if *I* were forced to explain their existence, would be there to standardize the language, to facilitate the learning and understanding of it between different speakers.
Can you imagine how confusing a language would be if there weren't any rules?
But the unique thing about grammar is that those rules are fluid, and change with use over time. Thus, your second description (Way to classify language), in my opinion, is an apt description. The thing is, you cannot use the "language changes" as an excuse to ignore the rules.
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