A benchmark is what you use for comparison purposes. So for example, if you wanted to compare the fractions 3/16 and 5/7, then you can compare them with the fraction 1/2 as the "middleman" so to speak. The fraction 1/2 is the benchmark fraction
3/16 is smaller than 1/2
1/2 is smaller than 5/7
We can see this if you divide up a number line (the region from 0 to 1) into various parts.
overall, 3/16 is smaller than 5/7
The order from least to greatest would be
3/16 < 1/2 < 5/7
In my opinion, it seems like an inefficient use of time. It would be better to directly compare the two fractions instead of bring in a third benchmark value.