They are unfit for any rational purpose.

Roman law gave a rational purpose for the existence of Frederick and his imperial ambitions.

No one quite knows why diners tip in the first place, or whether tipping serves any rational economic purpose.

Discursiveness and banality are intended for the rational purpose of conveying nothingness.

House criticizes social etiquette for lack of rational purpose and usefulness.

Aristotle did not believe nature is endowed with the same rational purpose and direction as human activity and artifacts.

"Then they could retain a rational purpose."

The tube just kept firing and firing, seemingly without any rational purpose except to destroy.

They say it's because the rational purpose of life is intelligence, and so when there's intelligence around, life develops faster.