This is an effective strategy because different phases do not compete for food.
For example for the case of Jelly fish, After a brief period floating about in surface waters like planktons, the larvae then settles to the sea floor, attaching themselves to one end. They then develop to polyps and begin to feed and grow. In spring the polyps bud iff immature jellyfish known as ephyra larvae which then grow to mature jellyfish.
