"Usage"refers to conventions of speech and writing that characterize groups of speakers. If you ask several people for some computer disks you'll understand them equally well whether they reply, "I haven't any" or "I ain't got none." Both answers are, technically speaking, grammatically correct, but you may make some inferences about a person based on which form s/he uses. Collectively, these conventions make up "edited standard English," the dialect used in most newspapers and magazines. This is the dialect you are expected to use in formal speech and writing. If you ignore these conventions in situations which call for them, your credibility as a speaker and writer will be damaged.