incumbent vs necessary

incumbent and necessary both are nouns.

Word	Noun	Adjective	Verb	Adverb
incumbent	Yes	Yes	No	No
necessary	Yes	Yes	No	No

incumbent (noun)	necessary (noun)
the official who holds an office	anything indispensable

incumbent (adjective)	necessary (adjective)
currently holding an office	absolutely essential
necessary (for someone) as a duty or responsibility; morally binding	unavoidably determined by prior circumstances
lying or leaning on something else