Answer to Question 1
ANY OF THE FOLLOWING IS ACCEPTABLE: 1) YOU DON'T NEED A LIST OF ALL THE INDIVIDUAL UNITS, JUST A LIST OF THE CLUSTERS; 2) IT CAN BE MUCH MORE CONVENIENT YET STILL ACCURATE; 3) IT CAN BE MUCH CHEAPER.
Answer to Question 2
ANY SITUATION WHERE THE SAMPLING FRAME IS HARD OR IMPOSSIBLE TO GET, BUT YOU HAVE SOME IDEA OF HOW BIG THE POPULATION IS.