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 English teacher 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 and wasting years of his life calculating meaningless numbers. The errors accumulated.
The same risk exists 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, one that just happened to have a similar name!
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 a reasonably exhaustive search for relevant records, correlation, analysis 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, the laws and culture of a region help them locate more records than you would find on your own, and perceive their significance more accurately.