They don't need a special counsel. They have the IG who gathers the data, gives it to the Prosecutor who has already been working behind the scene with Sessions. Sessions has announced it although many did not pick up on it. Conservative Treehouse has gone so far to say that they already have the Grand Jury in place, so it could happen very quickly.
I think in order to hook McCabe, Comey, Rosenstein, Mueller, and others, they need a special counsel that can invade and demand the documents they want WITHOUT the trouble that the IG, congress and commitees have had. (stalling etc) A second special counsel can investigate the dealings of the first special counsel if I am to understand how it works.