On gravel or road racers, I think a cadence sensor is essential. I also have a cadence sensor on my MTB. It happens less often with the MTB that I notice it's better to ride in an easier gear thanks to this sensor. The watt sensors are currently far too expensive for me, blatantly expensive stuff! I've already put such posh-posh pedals in my shopping basket several times and then thrown them out again. Unfortunately, I can't have a qualified opinion on this.

The one has nothing to do with the other. The cadence sensor is included in the bundle and is not expensive. Watt measurement is expensive and not included in the bundle.
I can't say whether it makes sense. Cadence sensor only ideal for the road approx. 80-100 revolutions per minute.