If installation costs per m² dominate because moderately-efficient panels already are dirt-cheap then using a more efficient panels that are a bit more expensiv

If installation costs per m² dominate because moderately-efficient panels already are dirt-cheap then using a more efficient panels that are a bit more expensive can save overall costs if you can get the same power with fewer m² and thus lower overall costs. And even if you drive down installation costs so you can roll out cheap cells en masse you'll eventually run into a space limit for some applications. There's only so much sun-facing rooftop on each building.