To the eye, a simmer is a gently bubbling liquid that comes from a relatively small amount of heat being used. By comparison, a rolling boil is a vigorous, bubbling boil with a sort of churning, active motion that comes from using a high amount of heat.
Other than when cooking pasta or blanching vegetables, there are few times when you need to cook something with a heavy rolling boil. Cooking with that level of heat breaks apart food instead of gently cooking it, changing the texture of the food in the liquid. The rolling motion of a heavy boil jostles and bumps the food around, knocking bits off the longer it cooks.
Typically, a liquid can be brought to a boil to begin with using a high level of heat, but once the cooking begins the heat should be reduced to allow the ingredients to simmer unless the recipe says otherwise.