Back in the mists of time, the village shaman said, "Bring me one of those shiny pebbles and you'll get a free hex-lift." As time went on, merchants and traders got more creative. "Get your free barley loaf with the purchase of a fine roast goose," they'd cry.
With only a couple of subtle differences, we're doing the same thing today. The question is, how do you sell just a bit better than the other guy? Use every means available to holler. That includes email marketing.