by-product vs need

by-product

noun
  • A side effect. 

  • A secondary product; something made incidentally during the production of something else. 

need

verb
  • To be obliged or required (to do something). 

  • To be required; to be necessary. 

  • To want strongly; to feel that one must have something. 

  • To have an absolute requirement for. 

noun
  • Lack of means of subsistence; poverty; indigence; destitution. 

  • A requirement for something; something needed. 

How often have the words by-product and need occurred in a corpus of books? (source: Google Ngram Viewer )