The reason to have a counterintelligence agency (which is the role the FBI has in the intelligence community) also be a law enforcement agency is somewhat obvious; domestic law enforcement, at least in that domain, is essential to counterintelligence. (I suppose there is an argument that the counterintelligence agency shouldn't be a broad spectrum law enforcement agency, but I actually think the dangers of an isolated, insular, counterintelligence agency and the kind of culture it would naturally breed would be a bigger problem.