If that standard heater is natural gas, then cool temps above the threshold give you the option to choose between heat or nat gas to heat your home. If power rates are cheaper, heat your home with the heat pump. If nat gas rates are cheaper, heat your home with the nat gas furnace. Thus you have some flexibility if the temps are in the threshold of being cool enough to run the heat, but not too cold for the heat pump.
That's how I've advised family members in northern climates who are getting old enough to need air conditioning in the summer. That's not how I do things with the usually warm Alabama climate, a variable speed heat pump, lots of incoming solar power, and, thus, an electric furnace that runs when the temps are too cold for the heat pump. But if I didn't have a significant amount of free power from solar, I'd probably prefer a nat gas furnace for when it gets too cold for the heat pump.
I have a heat pump (there is no natural gas where I live, and propane is expensive). It’s just a regular single stage Trane.
It’s capable of keeping my house at 68F when it’s 17F outside without needing the electric aux heat strips. That’s probably because my house is “tight”, not drafty, I made sure of that during construction.