The term usage refers to conventions, most often to those of language. Thus, "English usage" or "French usage" refers to the conventions of those languages, respectively.  When we refer to "word usage," we mean the conventions for using words; when we refer to "use of words," we mean only the employment of words:  "This text describes the principles of word usage."  "He is noted for his frequent use of wrong words."

People frequently use usage when they should use use.  The noun usage should not be substituted for use when the meaning is "the employment of" – even if you think it sounds more "sophisticated."  It is not correct to say that "the wise usage of computers saved the company money" or "usage of insulation can save fuel."  In both instances, use is the appropriate word.