From a 1921 new dealer ship opening promotional photograph. non-vertical storage bins. This dealership started construction in 1919 completed in 1921.
1. The owner must be very short.
2. The supplier has a great salesman.
3. No concern for square footage.
4. Theft deterrent.
That's a lot of wasted space.
The same reason why loose nuts and bolts and other unpackaged items - both large and small - are stored in bins or drawers at the hardware store: gravity.
It may seem like wasted space, but remember we are used to everything being boxed and stackable. All the parts in those bins are probably lose with tags on them. Which would be hard to store vertically. And if they are a dealer with a large inventory...
My guess is lighting. Tall bins would block the natural light. Look no light fixtures.
Donno. They look to be about the height of the rear fender on that car outside. Also the wheel at right might help with determining height. Don't think there are bins to remove but the compartment walls are removeable. Slots visible on the lower left in large compartments. Obviously some loner could have built them but I'm going for manufactured storage system.
Here is another from Ford Owner Dealer magazine, implies the dealers then made or had these made for parts storage. Fairly easy access too without stools or ladders