Why does the mormon have a reputation for having great knowledge of peoples ancestry? —monique...

what is the purpose of being able to do real far-fetching histories into peoples pasts?

Mormons believe in "sealings" that join families together in the hereafter based on ordinances done on earth in Mormon temples. So they do vicarious sealings for deceased ancestors. It's what keeps them going back to the temples after going through the first time for their own ordinances.

Ultimately they hope to vicariously baptize, seal, etc. every person who's ever lived so that each person may accept Mormonism in the afterlife *if they choose*. The Family History Library in Salt Lake is open to everyone, though, and I've never been asked about religion when visiting there.