First, it covered the never-repaired holes in the plaster from when, apparently, there was a leak in the bathroom above. At least they didn't just plaster over a Dixie cup, as the previous owners of my parents' house had done to "repair" a bathroom leak!
Third, it concealed the ever-pesky tops of the old cabinets.
And, lastly, and perhaps most importantly, it contained all of the wiring for the kitchen. Not just for the monstrously awful fluorescent lights in the kitchen as we'd assumed but also for the cute light above the sink, both light switches, the 3 whole electrical outlets which the kitchen contains, and who knows what else. The porch light and butler's pantry light could even be wired into this mess.
There's an older junction box that's actually above the plaster (you can kind of make it out in the photo above) which we might just be able to tap into entirely. Although it would likely require that we make more holes in the plaster to hook all of the wiring to it properly.
Also, we have the option of encasing the wiring more efficiently and then putting in a bead board ceiling higher than the dropped-ceiling was. But that seems like just a new band-aid to cover the old band-aid.
Our plumber recommends that we not patch the existing holes until we're finished with remodelling the bathroom upstairs, which could take years. Maybe we should just knock down all of the plaster, do it all right, and live with a construction zone kitchen for however long it takes to fix both the kitchen wiring and the bathroom layout upstairs. We'll probably go with the last option, realistically. But, in the meantime, we just have to ask our guests not to look up when then come into the house (the door in the last photo is the one we use for most coming-and-going -- it's on the street side of the house). Ah, the joys of an old house!