From what I understand, "legal advice" has a legal meaning, basically requiring it to come from your lawyer, and implying an attorney-client relationship. People want to be clear that they aren't a lawyer, and that they aren't attempting to practice law by offering someone else Legal Advice. Instead, it's more like Uncle Fred saying "You should sue the guy!"
Funny how this disclaimer is only found besides legal advice. Really, you should not have to write this. ("I am not a lawyer" is more reasonable, though.)