I also test modules for voltage required and different CAS settings to show how well a manufacturer has binned their modules, and how well they've optimized them for the best possible performance. Would tighter timings and a lower speed make this module perform better? Can we increase or decrease voltage and get more performance? When overclocking, there are many different considerations that must be taken and this test helps make a few of those choices.
G.SKILL always seems to bin their modules really well, with a fair bit of overhead. Changes to voltage didn't really affect these modules except in one specific case: I could boot into windows and run benchmarks for quite some time at each interval with just 1.5 V, but found that longer gaming sessions caused crashes with anything less than 1.65 V. Running CAS 6 was the one outlier in that it required a voltage boost to 1.7 V before benchmarks were stable, and 1.725 V before Battlefield3 stopped crashing. It is possible to run tighter timings and higher overclock speeds, but the default XMP settings are the best overall.