Think of concrete walkways. They are often done in panels or large tiles with expansion joints in between made of a material that can absorb the expansion and shrinkage of the concrete. Otherwise, the concrete would crack and break with changes in temperature.
Expansion joints are found in almost every piece of construction that has to deal with the elements. Some of them are clearly visible, others are not but they are all essential to for proper stability. However, over time expansion joints lose their resilience and stop being as efficient as they once were. This can lead to loss of structural integrity, and possible entrances for bugs and leaks. The damaged expansion joints can also be access points for further damage.