IG has no authority to prosecute a crime if he finds one. He can make an arrest but can't assemble a grand jury. DOJ shouldn't prosecute it themselves could create a conflict. So the AG brings in an independent prosecutor that can assemble a grand jury to avoid a conflict and do the clean up at the DOJ.
Everything about whats going on with the IG investigation has had a very tight lid kept on it. Sessions statement and the Nunes memo are the only things that are out there that can be speculated on. Sessions statement tells us there is an outside prosecutor working at the DOJ. Nunes memo tells us people have been questioned by him. The only investigation the public knows about at the DOJ is being carried out by the IG. Put the pieces together and the above appears to be what is happening. It's all speculation until the fireworks start exploding. Well they kind of already have started, there has been a major house cleaning at the DOJ.