To me, it seems useful in cases where the style is a part of the meaning of the symbol. This mostly comes up in mathematics, where a letter represented in fraktur or blackboard bold has some semantically different meaning, and this meaning can be part of the file instead of part of the foramtting of the file.

The practical part of me agrees with patio11, and that the knowledge gain of having these semantics inherent in the file is offset many times by possibly having to treat different bytes as the same character semantically.