There are certainly times when it is appropriate to use discrete surveillance methods. This will mean that people do not know they are under surveillance, so it can be useful for providing evidence or gathering reliable facts.
Discrete surveillance is often necessary with matters of national security. In these situations, recordings are often made of suspects to help gather evidence and prove or disprove whether they are a threat. Surveillance techniques are very advanced and it may be possible to place hidden microphones to record conversations that can be used as reliable evidence. Security agents may also be able to use hidden cameras and body cameras to record footage.
The main issue comes when the person in question suspects that they are being monitored. In this situation, they may be more aware of surveillance techniques and may seek out recording devices, making it much more difficult to monitor them discretely.