When Mario or Luigi collect one of these in Super Mario World, a Yoshi hatches and they can ride it. On Yoshi, they can swallow enemies with the X or Y button.
Yoshi also lays these eggs in Yoshi's Touch and Go and Yoshi's Island. In Yoshi's Island, Yoshi lays eggs by swallowing enemies. In Yoshi's Touch and Go, he lays eggs when he collects fruit. The eggs can be used to collect items and defeat enemies.