Necessity compels. In current usage this phrase is usually used to express something that is done unwillingly but with an acceptance that it can’t be avoided; for example, I really don’t want to cook tonight, but needs must, I suppose.

The phrase is old. In earlier texts it is almost always given in its fuller form – needs must when the devil drives. that is, if the devil is driving you, you have no choice. This dates back to Middle English texts, for example Assembly of Gods, circa 1500:

“He must nedys go that the deuell dryues.”