Guards, cleaners and maintenance staff see the patterns of the building: who comes, who goes, which flats are empty, when deliveries arrive. When you have a friendly but respectful relationship with them—knowing their names, greeting them, treating them decently—they’re more likely to notice and flag things that seem off around your door.
If someone unfamiliar tries to act like they belong in your flat, a guard who knows you might question them more closely. Staff might also be more comfortable informing you about suspicious behaviour in common areas.