Land is a finite resource, and not all land is created equally. Land that is near a harbor, in a city center, or very fertile is worth more than land that is agriculturally useless or inaccessible. Let's look at how each factor reduces the utility of the land.
Obviously active conflict zones reduce the usability of land because no one wants to be in an area where bullets and bombs are flying. The long-run effects are also significant. After the conflict stops, people may not return, either because they have established themselves somewhere else or because their homes have been destroyed or seized. Infrastructure may also be damaged. Land is less accessible, and therefore less useful, if bridges, power stations, or other utilities have been destroyed.
Natural and man-made disasters also make land less usable, often for similar reasons. A wildfire or flood may destroy crops and buildings. Repeated terrorist attacks may reduce investment in an area. As with the effects of armed conflicts, the amount of land remains but its utility is diminished.
Some disasters, such as erosion and climate change, even destroy land altogether. Coastal developments and certain mountain properties have always been threatened by water and wind erosion. Given the projected rise in sea levels, many low-lying coastal lands are expected to soon be below sea level. In each case, we see a reduction in the amount of land.
Finally, over-cultivation is when land is used heavily for a relatively short time but then becomes infertile. This is most easily seen in Africa and other areas where agriculture is fueling desertification. Clearly the transition of land that was somewhat fertile to desert is a loss of usable land.