One of the biggest advantages of prescriptive grammars, as vanka25 state above, is that such a prescription seeks and establishes a standard that leads to more efficient and effective communications. Adherence to the standard reduces ambiguity and misunderstanding especially when spanning local or regional dialects.
Another advantage of prescriptive grammar is that one cannot study the evolution of language without having a baseline. It is the prescriptive grammar that establishes that baseline since linguistic evolution is charted by noting the differences between any descriptive grammars and their corresponding prescriptive grammars.
I would also offer that adherence to the prescriptive grammar is far more important in the written word than in the spoken word, or in formal speech as opposed to informal speech.
The prescriptive grammar is also critical in the accurate understanding of historical documents. To understand what is meant based on what is written, one must understand the context and that most definitely includes the prescriptive grammars of that time of writing. In that same vein, what is written and recorded today will be viewed by the historians of tomorrow, not by the descriptive grammars of their day, but by the prescriptive grammars of our day.
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