Crown of England taxed wool by forcing it through one port

Wool was medieval England's most valuable export commodity, and by the 14th century, wool merchants shipped it out through a large number of English, Welsh and Irish ports, each operating more or less independently.

Earlier attempts to manage the wool trade had swung between forcing all export through a single foreign staple town (Antwerp, then Bruges) and abolishing staple restrictions altogether, with statutes reversed and re-reversed within years of each other as merchants, the Crown, and Parliament each pushed a different balance of free trade against fiscal control. None of these attempts had settled the underlying problem: a trade routed through many separate points of exit could never be reliably taxed at any of them with confidence.

Made the trade physically unable to leave except through one licensed port, so finite enforcement concentrated on a single choke point instead of spread thin across many.

Too many ports meant enforcement was spread thin, so evasion was easy.

A single legal exit point made all other routes illegal, not just under-monitored.

Concentrated volume at one point made inspection and taxation straightforward.

Licensing merchants at the chokepoint gave them incentive to police compliance.

When a flow is too diffuse to police, concentrate it through a single chokepoint you control, turning an impossible audit into a manageable one.

Concentrating the wool trade through Calais gave the Crown a single, fully staffed, fully inspectable chokepoint instead of dozens of loosely monitored ports, and customs revenue from wool became one of the English Crown's most reliable and heavily relied-upon income streams through the 14th and 15th centuries; the Merchants of the Staple, in exchange for the monopoly this concentration handed them, became major financiers to the Crown and helped fund the defense of Calais itself.