'Necessity' in the Bible

For when there is a change in the priesthood, there is of necessity a change of the law [concerning the priesthood] as well.

who has no day by day need, like those high priests, to offer sacrifices, first of all for his own [personal] sins and then for those of the people, because He [met all the requirements and] did this once for all when He offered up Himself [as a willing sacrifice].