it's like a rev meter in a car.
too high overtaxes your cardio and too low is no good for your knees.
cadence display will allow you to find your regular rhythm from which you can work on improving your technique or form.

aim for at least 80rpm.
most people average somewhere between 90 and 100rpm.
120rpm, and you might want to switch gears to the next one up.

don't bother looking at it until you've warmed up and found a nice stretch of road to maintain a comfortable cruising speed to find your normal cadence. If it's below 80rpm, you will want to work on spinning faster.