"What's the good of Mercator's North Poles and Equators, Tropics, Zones, and Meridian Lines?" is a quote from Lewis Carroll's "The Hunting of the Snark." It questions the practical use of the complex and detailed geographical boundaries and lines that traditionally help in navigation and mapmaking. The quote suggests that these lines are mainly conventions or symbols rather than inherently meaningful or necessary for understanding the world.

In the context of the poem, the Bellman would ask about the purpose of these lines, and the crew would respond by emphasizing that they are just conventional signs. This highlights the idea that some human-made systems and boundaries are symbols created for organization or understanding, but may not have an intrinsic practical purpose beyond their representational function. The quote reflects a playful skepticism about the significance of such conventions.