Rainwater is purer than tap water. Rainwater is originally free from chemicals, while tap water underwent chemical treatment such as using chlorine as a disinfectant. As long as rainwater is filtered and purified, it is safe for potable use, like drinking water. For watering your plants, rainwater is naturally designed to hydrate plants. Although both underwent treatment, rainwater remains more beneficial in terms of natural nutrients for the growth of plants than tap water.