The ICO will auto-adjust as you ride. Basically every time you get to a known mileage point you increment or decrement the ICO to match the mileage...this teaches the ODO a correction factor of sorts which is very handy. The cheaper ODO's will just spin at the same rate all the time. Why is this important? because the reality is that your front wheel slips and slides and brakes and sometimes is not on the ground and these all lead to distance errors. On longer events where you might have a couple of km between instructions the error can get quite significant.
My intuition nearly makes up for my lack of good judgement.
Just installed my new trolling motor