Why would you be descending at below cruise speed?
Let's say you are flying an a/c that cruises at 300 kts. You have been in the holding pattern at 210 kts and are descending in the hold to FL XX - you then learn that your destination has gone below landing minima and request a diversion to YYYY airport. ATC says "continue descent to FL XX, expect further climb on handover to ZZZ Control and proceed direct to WAYPOINT with no ATC speed restriction"
By the way, the autopilot is u/s!
btw the way another occasion when you might need to do this in a higher performance aircraft is after an engine failure/shutdown enroute - in order to driftdown safely due to high msa (eg over the Alps) you descent with max continuous thrust on the operating engine at the driftdown speed (typically in the order of 230kt) - as you approach level off the nearest suitable airport for diversion is 250 nm away - having cleared the higher terrain you now want to accelerate to a more economical cruising speed (which is faster).
Farfetched? Academic? Whilst I agree we are not teaching all ab initio pilots to go on to greater things, it is fundamental that they should understand and be able to apply basic techniques.