I will chime in here and say that in my experience it is not as much to let the dispatcher know, because a computer entry by an officer in the field will automatically change his or her status on the CAD ( computer aided dispatch ) screen that a dispatcher is always watching, it's to let all of the other units know whats going on in the district. The computer screen in a patrol car is constantly changing and an officer driving can not keep up with the changes on a busy shift.
However, all of the officers in the district can hear the radio as they drive and if they hear another unit making a traffic stop, they are aware of the stop and can ride by to make sure the officer making the stop is fine. Additionally, while many agencies have computers in most of their marked patrol vehicles, many agencies have not yet been able to equip all of their unmarked vehicles with computers, and even unmarked vehicles that have them, the operators are not assigned to patrol and therefore are not visually monitoring the dispatch status screen.
Finally, many officers assigned to work a shift together get to know each other's quirks and voices etc. Often, fellow officers can just tell by hearing another officer talk on the radio that there is just something "off" and that they should do a ride by and make sure their fellow officer if ok. Ya can't do that by reading a computer screen, only by hearing ya squad mates voice.
Back around 2000 many agencies tried what was touted as "silent dispatch" - not voice broadcasting assignments, only assigning units by computer screens in patrol vehicles only to discover all the pitfalls of that.