Something that is compulsory or mandatory in nature.

An action that needs to be taken

Everything in the past, including war was necessary, because it existed. This does not mean to say that it is necessary now or in the future.

When one has tickets to an NBA, NHL, or MLB playoffs, but the game they had tickets to becomes cancelled, as one team had already won the series. Only used for Game 5's, Game 6's, or Game 7's, the games that are scheduled as "if necessary," the derivation of the phrase.

A word you use sometimes but never spell it right

A word you say on a daily bases, but don't know how to spell

Not liking something but understanding that it has to be accepted (sometimes) or it has to exist.

An unwanted boner used for the express purpose of shutting some dumb bitch up.