I looked it up out of curiosity. In Buzan's "The Mind Map Book" (and other books by him), he suggests mind maps can significantly enhance learning, memory, and creative thinking, and that labeling lines/branches:

• Creates meaningful connections between concepts

• Allows more fluid and associative thinking

• Allow for greater creativity and recall compared to spider diagrams

He suggests using curved lines, colors and images, and to develop your own personal style of mind mapping