The phrase "necessary adjunct" means something that is needed or required to be added to another thing in order for it to work properly or be complete. Full definition
Explanatory parentheticals were no big problem: They just lengthened the footnotes in which source materials appeared, and they became necessary adjuncts to most citations. (abajournal.com)
Movement Therapy has been shown to be a positive and sometimes necessary adjunct to therapy. (fullcirclefamilycounseling.com)
In the right company, the right sort of drink is a necessary adjunct to the collegiate way of life. (firstthings.com)