Hypnosis involves concentrated focus of attention, heightened to allow the recall of those memories which we find can be elusive to the conscious mind. When the subject is in this heightened awareness and the critical factor is bypassed the subject becomes less subject to judgement which could cloud their memory.
If you had three people who witness a robbery and the robbers jump into a car and speed off each witness could give a different description of the events. One may say it was a Blue car the other a Green car and yet the other says it was a Black car and we can see that the information can be both confusing and inconsistent. The result will be that the investigation could be slowed down.
Forensic Hypnosis has the ability to form a more accurate description by enhancing the witness' memory and so enable the Primary Investigating Officer to proceed with their investigation at a faster rate.