oh man, just when i finally think im over wanting one of these you go and bring them up!

line counters are extremely useful tools when paired with other items to easily and repeatably target specific water regions.

in union with a line counter, you can use dive curves from manufacturers to know how much line to let out for a given bait to achieve a certain depth. lures such as the berkeley flicker shad (gross) have the dive curve right on the package. other lures have their curves available via precision trolling, through their book or the new app.
  (You need to Login or Register while expensive if this is something you would find useful it may be worth it. i am not overly familiar with the information because i am more of a trial and error guy myself (read as: cheap and doesnt own it). i think for all of their calculations they use 10lb xt (.305 mm diameter) and 10lb fireline.