If you file a complicated tax return and the "prepared by" line is not a CPA, you get targeted as someone who probably did something wrong. The IRS is more likely to go for you.
Sorry I assumed my question implied that was what was being asked. What is the complexity because I’ve done my own taxes for two decades, every year things are nuanced and slightly different. Never have I used an accountant.
I send my bank statements, credit card bills and other documents to my accountant. He produces a return, together with optimisation advice for the coming year. It’s well worth the money. (I also have real estate, private equity, fund interests and emerging trust situations, and am multi-state.)
1. You don't have to figure out which forms to file by yourself. This quickly becomes non-trivial if you run a business, take a lot of deductions, or have weird forms of income.
2. The IRS sees that your return was prepared by a professional, so it is much less likely that they will try for an audit (since a professional will be representing you).
If your tax liability is under $100k and your return is under ~20 pages, you mostly don't need one, but an accountant also doesn't charge much either (about the same price as TurboTax).