That curious little cable
There are actually a few uses for these cables. The manual suggests one: wireless triggering from cameras that lack a hotshoe but have a PC sync outlet; eg medium format film bodies. But also, with a 2.5mm micro jack to 1/4" plug adapter, you can connect the output of one of the receivers to the sync input on a studio strobe or monolight and fire that wirelessly, perhaps at the same time as one or more speedlights in a complex setup.
|Now, if only my meter had a hotshoe mount on it ...|
Trigger those flashes directly from your light meter
Doing this simplifies the two-step process of first arming the flash meter then manually triggering the flash(es). Instead, all you do is push the measure/trigger button on the light meter; the flashes fire, and you have your reading.
This works especially well if you have a spare trigger as you can leave it connected and dangling from the light meter. This means not having to worry about forgetting to replace the trigger on top of the camera like I seem to do over and over during a busy shoot where I'm frequently altering the lighting setup and needing to re-measure.
Step by step
So for those of you not used to this, here's what you do. You will setup your lighting arrangement, stands, umbrellas, softboxes, etc., making sure to pop wireless receivers onto the hotshoe feet of all your flashes and turning them on. Set your camera on manual, dial in for example, ISO 100, 1/125th sec shutter and set the flash meter to match those settings. Then when you are ready to measure the light to get the correct aperture, you merely hold the meter near your subject's face with the dome toward the light and press the Measure button. POP! (that's the flashes going off.) Now you will have the correct aperture to use on the meter display.
Aren't you glad you didn't throw that little cable away? You did? Whoops! Well be sure and keep the next one.