Structural work encompasses a wide range of activities that range from seismic upgrades, foundations for new structures, to foundation corrections and repair of water and insect damaged walls and ceilings in older buildings. Safety (preventing collapse or damage) and cost (eliminating future, larger repair) are the main reasons the work is done.
Much of structural work is never seen. It is covered by by cosmetic work (plaster, stucco, wood work etc.) Matching the new to the old is very important.