It's one guideline to use as you wish. To me, it helps me know more about players on other teams that I don't see play very much, if at all.
I see JBJ play great D, but I have no way of knowing if he is the best, top 10 or top 20. I have a pretty good idea how he stacks up against other AL East CF'ers, but even after seeing 18 games of them, I'm not that sure about my own opinion when it comes to comps... certainly no better than I do with dWAR numbers, UZR/150 and DRS.
That's all I'm saying.