Why should you go to one if at all possible? Well, because you can learn a lot attending your pick of the dozens of classes offered; you can interact with genealogists from all over the country; you can tour the huge vendor area; you can see a few sites (and possibly eat at a few great restaurants) in the host city; and you will definitely have a great time:
Note: I certainly agree with the author. I went to the NGS Conference in Kansas City in 2008, and had a great time! I would have been glad to go to Raleigh this year if the economy hadn’t taken a header (I wouldn't mind going to the FGS shindig in Little Rock later this year, either).
Speaking of national conferences: in November 1992, Joy Reisinger of Sparta, Wisconsin began work on an index to the publications (programs and syllabi) for the annual conferences of the National Genealogical Society (NGS) and the Federation of Genealogical Societies (FGS). In 1993 the organizations published the results of her work. After the Index to NGS and FGS Conferences and Syllabi was published, she then continued her work with FGS and NGS conferences and added many other conference years to her database as well. You can search her database here:
Hint: It's a terrific resource if you present genealogical programs or if you are in charge of securing programs for your society!