Well with cadence you get... well... ah.... er.... your cadence <grin>
You don't have to have it, but it does give you another number to look at and compare your data from several of the same segments of multiple rides. IE, a hill you might always try to climb at a high effort. You might see trends that show you doing better with a higher speed at a particular cadence.
I rode for a couple months recently without cadence. I missed it and put it on my new bike too. I'd been using it for more than ten years with my other bikes and knowing what I'm doing at any one moment seems to help make sense of the other data.
I'm sure there are those that will tell you that all you have to do is count your pedal strokes and do some math in your head. But cadence isn't a constant for a ride. You'll be shifting gears and constantly changing cadence. How much math do you want to do on a ride? <grin>