I cannot personally think of any examples off hand where no cassus belli was ever cited.

It was almost universally accepted that if you intended to rally your men to war there was going to have to be a cause or intent behind it. The reason or rationale could vary heavily and some were more tenuous than others, but the writings of St. Thomas of Aquinous weighed heavily on medieval rulers, with his concept of a just war.

But more practically speaking, you are compelling men to battle and they must feel as though there is a reason that they are risking life, limb, and treasure in order to go and fight.