Most building materials are dimensionally unstable.
They expand and contract under normal operating conditions. Such movement may be small but it can have very significant effects. In a building it is also necessary to join materials and there may be a gap between them that could let moisture, air or other substances pass through. Usually it is desirable to prevent this so the gaps are filled with a sealant. Joints, and the sealants used in them, must fulfil some simple but quite demanding requirements.