Around here it's fairly common to find service panels with two breakers, one for the air conditioning (usually 50A) and one for the 100A subpanel feed that handles everything else in the house. Often the AC's spot has split stabs so you can quad it up with your solar, while the 100A spot is solid stabs. You can find whole developments like this. They typically have 125A rated service conductors and service panels. My impression is that they date from an era after AC became standard in new developments, but before 200A services did.
You can see where the savings could come from in volume. Cheaper to have the lighting and outlet panel in the middle of the house so the average wire runs are shorter, but not for the higher amp AC circuit which is always going to a destination on the outside of the house, probably not far from the service panel. It just happened to work out for us solar guys thirty years later.
In California we also now have 'solar ready' stuff going on in new developments, by state regulation and incentives and as a selling point. So I've run into a couple new developments where all the service panels are Homeline MLO with space for 6 2-pole breakers and an admonition on the label not to put more than 6 handles in the thing (because you could, with some single pole breakers). Another section of one of the same developments had 225A busbars and conduits pre installed from the garage to the roof.