Importance, take Greece for example temples to the Greek Gods fell into disuse when Greek Orthodox Church became the main religion and therefore that church wanted the money from worshippers to build its own places of worship and would discourage belief in what went before. This allowed older religions structures to fall into disrepair (much like the in the uk when monasteries were dissolved by Henry VIII) helped to highlight they were failed institutions.

However it’s worth noting that in Italy and Greece there is evidence in history that earthquakes led to much of the initial damage leading to the buildings being unsafe and too expensive to repair and from there the time weather and deconstruction did the rest. obviously really tall structures still standing are much harder to steal stone from than the bits that have collapsed which is why some bits still stand and some bits are gone completely. We must remember though that some of what we see today in the colosseum is due to reconstruction works done over the last hundred years.

Buildings like the colosseum in Rome once damaged simply fell out of use because they had no further use, they were to big to take down and it was easier to let them decay. The once flooded and abandoned parts of New Orleans or the car factories in Detroit are chilling reminders that abandonment is something that even happens today.

All of these examples and places like the great pyramids are structures that fell out of use and much of their decay following their abandonment was because local people used them as sources of building materials almost like a quarry only the stones were already cut and piled up.

Remember with many of these old buildings disuse allowed soil/sand to build up and structures slowly being lost under the ground and out of sight. much of the ancient Roman architecture we see in Rome today was only “rediscovered” through excavation in the 18th/19th centuries.