We want to have all information about secret places of detention because whenever there is a secret place of detention, there is also a higher risk that people are subjected to torture.
Fact-finding on the spot has to include interviews with detainees. What's the sense of going to a detention facility and doing fact-finding when you can't speak to the detainees? It's just nonsense.
Every secret place of detention is usually a higher risk for ill treatment, that the danger of secrecy.
Among them is that prisoners are exposed to extreme temperatures, or long detention in isolation.