"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.
Sarah: I think global warming is a farce.
Tom: Are you stupid? Global warming is uber real.
Sarah: Well according to this study, global temperatures have stayed the same for the past 10 years.
Tom: Uhh... hmm... Not necessarily.