Something that is compulsory or mandatory in nature.
An action that needs to be taken
* It is not necessary for Ryan to lick my ass.
* I find it necessary to use sexual slang at least every other sentence. It makes me look mature...
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.
People started to realize the basic game center could move to life support instead of death support and it was now necessary to work for only 6 hours per year instead of the current 2,000, to support war mongers.