Doesn't take much electricity to kill a fish in water.
My first guess would be that since synos come from a land filled with mormyrids, this would be a defense for them.. Even though a mormyrid can't shock fish, they use weak electricity to see their prey, and I'm guessing if a syno is insulated, the signal gets absorbed and doesn't bounce back? ? That's the only thought I would have.
Maybe in the wild they follow electric catfish for protection.
I'm guessing Electric cats also use their electricity to communicate, and if so, I was reading that elephant noses and knives and such can't breed in captivity because the signals bouncing off the tank walls are too hard for the other fish to read, so they can't tell each other they are ready to mate.