If the bush is looking healthy, is there a reason to cut it back to
the ground? Does it flower on new growth, or does the pruning
stimulate flower production? Or should I just leave well enough
alone?

Thanks.

-Fleemo

Buddelia and Caryopteris both bloom best on new growth.
Cutting back stimulates new growth.

Pencil thick cuttings 8" long root readily so make some new plants
from the prunings.