1. Expertise. (Which might mean saving money thru settlement (OIC) or other methodologies).
2. Reducing Hassle.
For some clients - the hassle factor, e.g., staying on hold with the IRS, setting deadlines, avoiding levies - is the reason paying a tax professional makes sense. They value their time and don't want to deal with the IRS in their spare hours.
For others - who have complex tax problems or who hiring an expert might mean the difference between paying tens of thousands of $$$ OR Not.
But let's be candid - if you have a back tax issue - and the result of negotiations is going to be a PAYMENT PLAN and NOT an Offer in Compromise -- then hiring a tax professional is to save hassle. And for that reason in many cases - you might be better off - value-wise - NOT hiring a tax professional.
A few years back we wrote a piece 7 Reasons to Pay for Tax Representation to further illustrate these points.