The members of the Identification Unit need to collect all the physical evidence that may lead to someone being convicted of the murder. It is a painstakingly slow process. They don’t want to overlook something that might be essential. When these officers have to answer questions in court six months later (or more), they need to have the information, the evidence and a rationale for what they did when questioned relentlessly by the defense counsel. 

It is also critical to protect the crime scene from contaminating materials which might come from outside the scene. Sometimes the members of the Identification Unit wear special suits which they may change several times during one day to prevent such contamination. The Identification Unit wants to guard against the possibility of accusations during the trial that the scene was contaminated by outside materials.

Access to the scene is very restricted. Officers who have access to the scene are not allowed to have contact with the accused since the defense could suggest that a police officer transferred evidence from the accused to the crime scene.

Mennonite Central Committee Canada. (2011). Getting through the maze: A guidebook for survivors of homicide.