I know what they are and how they are used, what I don’t understand is why as they are motorised, the rider continues to pedal them.

Pedal assitance only, if the rider stops peddling then the motor isn’t powerful enough to keep them going, or the used to be that way.

So you can vary the speed very slightly.
If it used a motor alone, the acceleration would be too great, it needs to build speed gradually to allow the riders to stay behind it.

I would’ve thought that a sensitive throttle and a slightly more powerful motor would be the answer. So the rider has to be reasonably fit and not a lardbucket?…

Seems a perfect job for an electric assist bike really.

So the rider has to be reasonably fit and not a lardbucket?…

Lardbuckets are better – more space to hide behind out of the wind!
The Derny guy isn’t there to be fit, he’s there to pace the riders and the Derny is just traditional. Electric motorbikes would be better (the GB track squad use one for their pacing) but they’re much more expensive and given that all tracks have a fleet of perfectly healthy Dernys, it would be silly to replace them.

Seems a perfect job for an electric assist bike really.

Yes but it’s about tradition too.