The construction of your property comprises of two walls separated by a cavity. These walls are tied using different types of wall ties, butterfly, fishtails ECT.

Most homes built in the last hundred years are of cavity wall construction; the external walls are built of two 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 houses 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.

In the 1980's it became apparent that the wall ties deteriorated much quicker than expected.
Leading to cavity wall tie replacement being required.
According to the BRE.  10 million homes in the UK may already have the problem however  yet to be diagnosed.
There are many Issues associated with failing cavity wall ties, from horizontal and stepped cracking of the masonry  mortar joints, and bulging of the walls