"Not necessarily" is a figure of speech used to dispute a statement without having to explain why. It's typically used during debates by cowards who are more concerned with their opponent being wrong than they are with themselves being right (and/or) by people who always have to have the last word in any discussion.

Although the statement can be used before making a valid point, sometimes pretentious people will use the phrase by itself. This effectively allows them to appear correct without giving any evidence whatsoever to prove the validity of their point.

A thing that Krisiic (a.k.a. King Lurka) says all the time.