Two stage heat pumps are more efficient at lower temperatures. Like heat pumps in general, they're based on state change. Finding the right 'medium' is a matter of finding something that can condense/evaporate around the to/from temperatures you're targeting. It's possible to 'juggle' this by changing the pressure (usually it's vacuum - fluids evaporate at lower temps as pressure goes down) but there's a limit to how far you can extend the temperature differential between 'condense' and 'evaporate'. If you stack one circuit on another you can sidestep the issue.

The downside of course, is that they cost more; not as much as ground source, but more.

We looked at ground source. As our house is forced hot water (AKA 'hydronic') through baseboards, we'd have to have ducting run - and that actually costs *more* than drilling for the ground loop. Sigh...