I know who my ancestors were back to about 6 or 7 generations -- why would I want to know more? Oh, sure, I'm about 10,000,000th in line for the throne of England and even closer than that to be Holy Roman Emperor, but the number of people I'd have to knock off to take either office is pretty daunting.

I don't see any reason to do it, unless a) there is some Awful Genetic Disease involved or there is some other compelling reason or, b) you're so obscure you need this sort of thing to feed your self-esteem.