The 6 year metric is one that is collected by the Government in the Common data set. Along with any rankings or or lists it’s so important to figure out which metrics are important for you.
I do think the 6 years gives you a good barometer on % of students that start that actually earn their diploma. Personally, I found that for students looking at schools outside the top tier it is a good indicator (along with Freshman retention rate) of how well the school supports students that might be struggling. Back in the 90s, I think more of the accountability was put on the student to seek additional help when struggling. The schools that do well on those metrics seem to do a better job of things like academic advising, tutoring, mid-term grade reviews, etc.