The bitcoin message magic, "Bitcoin Signed Message:\n", is defined: https://github.com/bitcoin/bitcoin/blob/203632d20b3107009f45fae56cc3f7d828b16660/src/main.cpp#L80
I know it is used as a part of signing messages to prove they were sent from owners of specific private keys, as detailed here: What are the safety guidelines for using the Sign Message feature?.
What I don't know, though, is what good is this message magic? Does it make sure the same key can't be used in two coins? Is there some cryptographic reason?