You mean it's defined in the library but never used?
It's worse than that. Not only is 'cols' not used, but 'rows' is only checked to see if it is greater than 1, the actual value isn't used.
If they had used the 'cols' value then they could have made the 16x4 displays function correctly, right now they offset the third and fourth rows.
Another unusual characteristic is that the library defaults to a 16x1 display if you don't invoke LiquidCrustal.begin(). Those displays are relatively rare, most 16x1s are actually 8x2 internally and should be invoked as such.
I suppose the values are defined for future expansion such as that done by jrraines in his LiquidCrystal440 library.
Don