In order for termites to grow through the stages they must shed their skin. In the process they loose the protozoa in their gut. So a constant supply of the protozoa must be available to keep the colony healthy. The workers chew on the wood that will become the food. This is what causes the damage to wooden structural members.
Workers then go back down to the colony and feed the others.
Tubes are sometimes built on the outside of walls to bypass an area, such as a brick or concrete wall. Subterranean termites need moisture at least in the form of increased humidity. Sometimes they will bring mud up from the ground and pack it in the damaged wood to provide the moisture. Termites can travel hundreds of feet in search of wood food source. They are slow but persistent.