Why might an instructor in a small aircraft choose to descend with a substantial yaw on a first lesson?
Sounds like a Forward Slip. It is not an unusual maneuver. It is very useful in order to descend faster than normal (Vertical Speed) without increasing your airspeed. The maneuver reduces the amount of lift generated while at the same time creating a large amount of drag by presenting the broad side of the fuselage to the relative wind. 

Did the instructor happen to reduce the throttle to idle right before the maneuver?

It is also a great maneuver to practice in case a no-flap landing is needed due to flap failure.