If you're playing SimCity and throw down some residential zones, you might notice that they start out by putting up these little single-family dwellings

You can demolish them individually, which seems kind of pointless, really. But then you can put something in the hole, like a Police station.

Then you demolish the whole residential zone. The zone goes away, and so does the part of the building that was overlapping it.

It still protects the city as well as the whole building did, and you can put stuff in the extra space, like parks, to increase the value of the land. And with a little planning...

That block in the middle is a Fire station. It only takes up one block, but still protects as well as it would if it were full size.