Problem is that trace evidence is gathered using a focused vacuum cleaner, drawing what it picks up through a filter. The filter unit is then sealed until it is processed at the forensics lab. A Sheriff in Wyoming uses a forensic team from Cheyenne 150 miles away. With his scent transfer unit, he can gather and preserve scent at the crime scene where it would take the technicians hours to even get there. There is good reason to want to capture a scent sample as early as you can.
Scent is fragile under most circumstances, but it can be durable depending on conditions.
Connecticut State Police dog handler Sgt. Kevin Rodino reminds me scent-life is measured in minutes at a downtown crime scene, hours in most cases, days in some protected areas, and even month in rare cases. The earlier you can capture a sample, seal and mark it as evidence, and store it in a freezer locker, the better your chances of identifying the culprit in the distant future.