Is to compensate for a delay between "dating" somebody and making it an official committed relationship by setting the "official" beginning of the relationship at the first date or earlier. Must be agreed upon by both parties otherwise it can be considered psycho.
I have to go meet my girlfrieds parents but luckily we backdated our relationship and 6 months sounds so much more stable.
When you have a school project that you were told about six months ago, but you did it the night it was due, you backdate it by putting dating a day from earlier to make it seem like you've been working on it for a while. Also very useful when you're doing an assignment the day its due.
Joe had just finished his history paper second period on the day it was due; in the heading, he backdated it a week so that the teacher would believe he didn't do a half-ass job on it.