Before this question is answered it is interesting to note that rain will work about as well as your tap water will. But, that is not really the issue here. Even if you live in an area that experiences high amounts of rain every year, you still may need a garden sprinkler. The sole reason is that relying on the rain is too inconsistent and sporadic. If you think about it, it could rain for seven days straight for eight hours a day. Then, it could turn around and be spotless for two straight weeks. Plus, the watering of this method is not even and can even oversaturate your garden.
The growth and health of your garden will be compromised if you rely on Mother Nature to do your deeds. Now, while it is raining, there is no need to have your sprinkler going as the rain is a sufficient source of water. It is just that you can’t put all your eggs in that basket.