If you're ever involved in a criminal trial with the IRS, your accountant may be forced to testify against you. Just imagine the sinking feeling you would have in a criminal trial if you heard an IRS lawyer say "Your honor, we would like to call the defendant's accountant to the stand".
If you hire an attorney instead to assist you (and your accountant), you would never have to hear those words. Only with an attorney do you have "Attorney-Client Privilege". In other words, your lawyer doesn't have to tell a criminal court anything that you've told him. An accountant has no such privilege.