This post is also from a teacher of statistics. Sure, you can weigh the whole bag to get the total (or average) weight; how do you propose to measure an aspect

This post is also from a teacher of statistics. Sure, you can weigh the whole bag to get the total (or average) weight; how do you propose to measure an aspect of an entire population, say one as large as a country? Sampling has its uses, when done properly. Besides, as others have pointed out, most people are notoriously bad at estimating weight.