Cavity trays prevent moisture being carried from the outer wall to the inner wall.
Most common places this is found is, at an abutment of a roof, above doors and windows,
and above concrete slabs or beams.
Cavity trays can wear over time and need replacing, but on the other hand you some times find,
incapable builders do not bother to install a tray at all,
which will only lead to one thing, and that is water ingress producing damp.
Due to either age or poor installation by unprofessionals,
water ingress can cause damp,
and mould which can will damage the inside of a building.
Repointing Brickwork - Isle of Wight