You can accomplish quite a bit of genealogical research on your own these days, without leaving the comfort of your chair. There is a lot of information up on websites. So why hire a professional?
Let’s consider the lesson of William Shanks, a 19th century Englishman who spent 17 years of his life calculating the mathematical constant π, a laborious pencil-and-paper effort that yielded him, in the end, 707 decimal places. However, unbeknownst to Shanks, he made an error in his calculation after 527 decimal places, causing every digit beyond that to be incorrect. All of his labor after the first error was in vain. It was error on error. He wasted years of his life calculating meaningless numbers.
Similar risks exist in genealogy, of spending hours, months and years trying to extend a line back more generations, only later to find (if you find out at all) that you made an error earlier in the research, and have been wasting your time doing exquisite research… on someone else’s family!
A professional (a good one, at least) follows the genealogical standards promulgated by the Board for Certification of Genealogists, including the Genealogical Proof Standard, the “gold standard” for genealogical research. This standard is aimed at reporting credible findings, after undertaking reasonably exhaustive research, analysis, correlation and reconciliation of the evidence, leading to a logically argued conclusion.
Finally, a professional knows where the bodies are buried, often literally as well as figuratively. Their knowledge of the history, and the laws and culture of a region, help them locate more records than you would find on your own, and evaluate their meaning and significance more accurately.