That is based on the principle that probable cause may be transfered from one law enforcement officer to another.

If Fred (if he were still a sworn officer) told me that he developed probable cause to believe that Skinny committed the crime of teabagging an artifact reproducer who sold them as authentic, I could arrest Skinny without having to develop my own probable cause before I arrest.
Of course the easiest thing to do is for Fred to get an arrest warrant, but circumstances don't alway make that feasable, such as if Skinny is about to flee to Mexico and open a chicken farm.