Confused by Cartographic Conventions

Daniel Huffman writes that “there are certain cartographic conventions out there for which I don’t understand the logic.” (Such as that thematic or choropleth maps should be on equal-area projections.) “I do not suggest that these conventions are wrong; only that I lack a clear, intuitive rationale for following them, and so haven’t always incorporated them into my own practice. Maybe you can help explain them, or maybe you’re confused, too.”