If the population is large. Random sample of the items made by the members of the population may be difficult or impossible. So instead of using a random sampling of all units.
In the case of a large population. A random sample of the action by members of every unit of the population may be difficult or impractical. So instead of using sum from every unit.
In the case where the population is large, the random sample group is committed to the list every unit member of the population may be hard to do or do not, so instead of using the method to pile all units