This was suitable for use as Service Equipment. From your photo, the lower 12 pole section was for the "Mains" (before we started calling them Service Disconnects). Then, as now, there could only be a maximum total of six operations of the hand to disconnect the service from the load (up to 6 single poles breakers, 6 double pole or a mixture totalling to a max of 6). The size of the service conductors were based on the calculated load at the time of installation, plus any room for expansion that may, or may not, have been added.
I look at this photo and see it as upside down. I helped install a lot of residential split-bus panels when I was apprenticing.
Even with the "Lighting Section" bus being rated for tandem breakers, over time, I've seen these fill up, and the "open spaces" in the "Main Section" mistakenly (or deliberately) used for branch circuits.
As always, there's no accounting for those who will do anything to get the job done quick and cheaply.