It's basically a quick visual reference to check yarn WPI while spinning to ensure your getting the weight you want. However, I noticed a lot of comments here and there that its a good quick check when determining yarn weight of an unknown yarn rather than actually wrapping and counting. Since I thrift a lot of yarn and a lot of it has no label, this would speed up my identification process.
As a cheap, quick visual reference card, I just wanted a printable version. I found one (that's it...only one) that I could print but try as I might, I could not get it to scale. I then decided to make one myself using Excel, and it turned out great. I printed it at actual size and it matches up perfectly to sizing. I added a few things for easy reference based on Ravelry's Standard Weights and voila...a super handy reference which can be printed and laminated.