Question about Garmin Speed and cadence sensor 2

If there is no GPS reception, the speed and therefore also the distance will continue to be displayed if the speed sensor is present. However, the wheel circumference should be determined as precisely as possible so that the display is accurate.
The cadence sensor could be used to train according to cadence. Both are not mandatory and are nice to have

Cadence lets you know your pedalling rhythm so you can adjust it to suit your training.
The speed sensor measures wheel circumference and is more accurate than GPS.
It's up to you to decide whether you need (or want) it!