Sometimes, with music and/or conversation in the car, a driver can miss the early sonic warnings as the car is entering a radar field. Having someone else able to see the display can increase the chances of catching the alert early. Also, I drive without my reading glasses, and discerning the tiny directional arrows on the display can be a little difficult sometimes. Another pair of eyes can help. (The Radenso also gives the direction from which the signal is coming by voice alert, but it only does so once and, back to the first point, it can be easy to miss.)
By the way, our installer just got back last night from the annual Mobile Installers convention in Las Vegas. He was sporting the first-prize trophy for Installer of the Year, based on detailed video submissions of the installation procedures and end result of his work. I've had radar installations done in many places over the years, including Los Angeles and Chicago. This small one-man shop in out-of-the-way Bonita Springs, FL is easily the most elaborate setup for this kind of work I've ever seen. People ship cars in to him from all over to get complicated installations done -- which is why you have to reserve a spot months in advance.