If you don't need a phase coupler now, you will at some point. Most houses have electricity coming into the house at 220 Volts. This is then split up into 2 120 V legs. Some circuits go on one leg and some on the other. Your x10 signals can't travel between the two legs of your wiring unless you couple them. Maybe you've been lucky so far and only been installing devices on the same leg.

Martin
Martin@AutomatedOutlet.com