The general answer to this is either to save you time, to save you money, or to take advantage of the particular skill-set of a professional researcher; and often the answer is “all of the above.”
You may simply be too busy to accomplish one or more of your research goals. You might live too far away from the location of the documents to be searched in order to make it economically feasible to do the research yourself. Or you just may not know where to look, what to look for, or you might not be sure of how to properly evaluate what you find.
These are all good reasons to use a professional researcher. In fact, you might be surprised to know that professional genealogists use each other from time-to-time, and for exactly the same reasons outlined above.
