Letters of Marque is valuable to aggressive maritime civilizations (especially the English, the Germans, and the Ottomans, all of whose unique units are naval raiders). Make liberal use of Privateers and Submarines if you adopt it - otherwise, the penalty to Trade Route yields can make it hard to keep your economy solvent.
A Letter of Marque and Reprisal is a document, issued on behalf of a government, which gives the holder the right to take commercial shipping of an opposing nation during a time of war. Those who carry these letters are privateers, as distinguished from pirates (because piracy without a nation backing you is illegal). Letters of Marque were an extremely popular tool of military policy during the late 18th and early 19th centuries, and could be a lucrative investment for backers of a successful privateer. The value of a captured prize was divided amongst the privateer’s backer and crew, and could be quite an enormous sum, which tended to attract seasoned sailors, drawn to the prospect of making a year’s pay in a bright and sunny afternoon.
During the War of 1812, the saucy American crew of the brig “Chasseur” managed to capture British prizes in the English Channel, and even publicly declared herself enforcing a one-ship blockade of the island—which had no real practical force, but enraged the Royal Navy, terrified the London merchants, and delighted the citizens of Baltimore. Indirectly, their actions also led to the White House being burned down, but we’ll set that aside for the moment and admire the chutzpah of the act.