At least in Europe, much of the current expansion is fueled by the parallel rise of PV solar installs in households (even without subsidies, both combined result in cheaper heating than gas).
The technical distinction is important, because you get out more heat than you would by just running the electricity into a resistor. And not just by a little: you might get 3x as much heat out.
It's not magic, though it sure seems as if it would be. You really are extracting heat from the outside; the outside gets even colder.
Electricity is involved, but to drive a mechanical motor. It's thermodynamically different from pumping hot water around, where 1 joule of fuel turns into 1 joule of heat. In a heat pump 1 joule of electricity can turn into 3 to 5 joules of heat.
The heating system in place was good enough. It used existing infrastructure.
Heat pumps are great for new construction, and they're even more important now that you're adding air conditioning. You didn't need air conditioning nearly as much a couple of decades ago.
Heat pumps are very common in US homes. One advantage is that they run off electricity rather than gas, so you don't need to add a gas hookup to the house. Existing European infrastructure is mostly already rigged for gas, so that's not as big an advantage.
Even now with 10+ years to break even on the installation it's hard to justify if you think you might move in the next decade.
Here in the UK it would cost £7k-£14k for an air source heat pump, and you'd get between £200-300 reduction on your bills.
If you are moving from a fossil fuel heat source, rather than electricity, then you do get a £7.5k grant. But if not, fuck you.