Asking "why do we need lawyers" is like asking "why do we need CPAs?" Or "why do we need programmers", for that matter. In modern society its just the product of division labor. Some people specialize in the complex field, then advise other people. They leverage the economics of the fact that their knowledge can serve multiple clients who have similar problem, so the clients don't have to learn the law themselves.
It's not the phrasing that makes the law complex. It's the fact that the law is a mechanism through which we mediate human interaction, and the interactions of modern society are phenomenally complicated. E.g. the Sherman Antitrust Act of 1890, which is still basically "the law" in the area of antitrust, fits in just a few pages: http://www.linfo.org/sherman_txt.html
It's not dense legalese. "Every contract, combination in the form of trust or otherwise, or conspiracy, in restraint of trade or commerce among the several States, or with foreign nations, is declared to be illegal." That's the gist of the Act. Yet, many lawyers make their living specializing in this law. Why? Because restraint of trade is hard to define precisely (if the Act tried to define it precisely, it would be dense legalese, like the Tax Code). Courts navigate antitrust suits largely be looking at what they've done before (consistency of result is often more important in law than getting the 'right answer', which may very well not exist). Lawyers help their clients navigate through that web of precedent.
It used to be the case, a hundred years ago, that law was unnecessarily complex. You had to plead cases in this highly stylized format, etc. That's all gone now. Courts will go to great lengths to make sense of a complaint written in crayon. What's left is a lot of necessary complexity. Look at the Federal Rules of Civil Procedure: http://www.law.cornell.edu/rules/frcp/#chapter_vii
Again, it's not dense legalese. "A summons must be served with a copy of the complaint. The plaintiff is responsible for having the summons and complaint served within the time allowed by Rule 4(m) and must furnish the necessary copies to the person who makes service." When law students take civil procedure, the greatest source of complexity actually ends up being complexity inherent in our federal system: choice of forum, choice of law, etc. Under what circumstances can a California court exert jurisdiction over an Alabama resident? When does a federal court have to apply state law? Which state's law?