I have a set of "passing lamps" on my bike, and the way it is wired, I have to physically turn them on and off with a switch located on the right handlebar directly under the power/kill switch. In the case of my bike, the hi beams do not make any difference as to whether the passing lamps are on/off. I can have both on or both off, etc.
To be honest, I don't know what they are "for", but I turn them on when I want to be just a little more visible, whether in daylight or at night. Basically, I turn them on in heavy traffic whether I'm "passing" or not.