A gang member being shot down is pretty cut and dried - straightforward.

There may be something very unusual about this murder that they want to keep quiet - details that only the killer would know. This is VERY common when an odd weapon was used, there were crimes committed in addition to the homicide, or just simply something really out of the ordinary.

It's also required when there's a strong piece of evidence pointing to a specific occupation or skill that might warn a suspect.