From time to time, an empire may need to shift its focus to better cope with its situation - from here, you may spend time demolishing structures to make room for new structures to build.
Demolishing structures is slower than building (due to the difficulty of disposing of the remains), but some of the original costs can be recovered.
Unused acres of land can also be dropped from your empire, though the need to relocate citizens and troops, not to mention the paperwork, makes this a slow process.