I ride in Arizona, so lots of bright sun means it can be hard to see the display- the audible beep when a car first comes in range is a great help.

When it first detects a vehicle, it supplies an audible beep, which is good if you’re in the middle of a standing climb or not paying attention to the monitor.

When a car is detected, the unit will give an audible beep, and a red bar will show up on the left and right side of the display with a small dot that represents a car, if the unit detects two cars, two dots will show and so on

If there's always someone behind you then this would actually become annoying (constant beeps) and irrelevant.