That's the level where humans begin to not be able to locate the source of the sound (localization). It also takes some of the pressure off of your receivers amplifier.
@lovinthehd likes to set his higher (I believe 100hz right now?), but he has 4 subwoofers and no localization issues. Some people might have big speakers capable of playing low so they might set the XO to 60 or 40hz.