What is the most useful facet of the "metric" system? what is the reason for giving equivalents like the mass of one gram is equal to a large paperclip?

I would say, as a longtime user of the metric system in mining and geology, that the most useful aspect of it is probably that it is a decimal system ie that it is all relatable by dividing or multiplying by 10's, 100's or 1000's mostly. For example, a centimeter is 1/100 of a meter, and a mm is 1/1000 of a meter and a mm is 1/10 of a cm. In map scales, say 1:2500 means that 1 cm =2500cm or 1 cm = 25meters so this makes it easy to use. Using equivalents like 1 gram is = to the weight of one large paper clip is to help people most likely who are used to the Imperial system to visualize the metric system to things they are familiar with and thus feel more comfortable with it.

It shows that the experiment is designed properly.