A good camping knife is a necessity if you’re going to be hiking in a heavily forested region. A necessity is something that you must have in order to complete a task.
When used in the plural, necessities are items required for a situation but nothing extra. You probably only bring the bare necessities to a sleepover — pajamas and a toothbrush. Sometimes a necessity is a thing you must know how to do. Knowing how to balance your checkbook is a financial necessity. In some cases, necessity simply means "need." Do you do your homework out of necessity or because you want to?