Following up on @snips-n-snails.
The use case for this is to organize printouts for reference. A simple example is that if someone wanted the ID of a vendor or customer, it was not untoward at the time to not look it up on the computer, but, rather, pull out a printout and look it up there. This organizes printout.
You can easily envision a printout for the Customer Master (sorted by name) to find a customer code by name, then grabbing the Open Accounts printout to find the customers current balance.
Part of the problem was simply storage space. With floppy based systems, and the small floppy sizes of the time, there simply wasn't enough space to keep the data on line all the time, and it's much safer to dig through a printout than have someone swapping floppies etc.
Finally, of course, it takes very little training to learn to navigate a report, so it's easy to stick one of these up by the front desk, away from the computer.
This storage cabinet is simply a manifestation of the day of folks making do with what they had. Today, naturally, we take this all for granted.