Lawyers generally assume that non-lawyers cannot compete with them by giving legal advice. That, the Bar would say, is the unauthorized practice of law. Rules against the unauthorized practice of law can protect consumers, but they also keep prices higher than they'd otherwise be, at least in some segments of the legal marketplace. And, then, there's that pesky First Amendment. Why doesn't a non-lawyer have a commercial speech right to give legal advice? And why doesn't a consumer have a First Amendment right to receive that legal advice? The answer the Bar would give is that the First Amendment, particularly in the commercial speech realm, is not absolute, and licensing lawyers protects consumers by assuring that only people trained in the law (and the law's ethical rules) give legal advice.
But the rules are not consistent. Law people may represent people in many administrative law contexts. For instance, non-lawyers can represent clients in social security disability hearings, though often those cases involve a complex mix of law, fact, and medicine. But, yet, in many (perhaps most) states, non-lawyers cannot represent clients in any court case, including, say, in a simple divorce.
What about licensure of people other than lawyers? Read this interesting article by Paul Coakley, a lawyer representing someone who provides dietary advice to people via his website. Coakley's client, Steve Cooksey, became the target of the North Carolina Board of Dietetics/Nutrition, which says that Cooksey's website constitutes the unathorized (and criminal) practice of dietetics. Coakley says that Cooksey's website is fully protected by the First Amendment.