You get acclimated to your 1 or 2 regular lines and then when you go exploring, this would come in handy. But, even on your regular lines, if you go to an unfamiliar stop this would be handy (the A/C line has 18 possible stops on my commute - I use 3 of those regularly and many others infrequently enough that this would be useful). The other scenario is when you move or switch jobs, your subways lines may change so this gives you a head start on the learning curve.