Most homes built in the last hundred years are of cavity wall construction; the external walls are built oftwo leaves of brickwork linked with steel ties. This makes for a warmer and drier home.

But, unless the ties are of stainless steel, or very heavily galvanised mild steel, there is a tendency for the ties to rust making the wall less secure and sometimes causing bowing of the brickwork.
Characteristic signs of corrosion include horizontal cracking of the joints in which the ties are laid, caused by expansion of the metal during corrosion.
Mild steel ties have a shorter service life than the brickwork in which they are placed and, during the life of the building, may need replacement with stainless ties which, because they do not corrode, will ensure a permanent solution.

Most homes built in the last twenty years will have been constructed of stainless or heavily galvanised ties which are proof against this decay but houses prior to 1980 cannot be presumed  to be immune, hence the need for inspection.