Do you know the difference between a subject and a predicate or between a compound sentence and a simple one? How about the differences between capital and capitol, color and colour, action verbs and passive verbs? Do your subjects and verbs agree?
Learning how to diagram a sentence will not teach you to be a great story writer, any more than learning to read music will guarantee you fame and fortune as a lead singer. However, understanding grammar will show you how English language works, which is certainly handy for every writer.