A centuries-old tradition, why cats are kept on warships

Because there will be rats on warships on long voyages, and the rats will eat the food on the warships, especially since the rats reproduce quickly and will consume a lot of food on the warships. In addition the rats will also make holes in the warship (windjammer) era, damage the warship. So keep a cat to catch rats is a lot of warships on the inevitable choice of anti-rat.