The best reason I can think of is if the idea is to improve traffic flow in general in favour of the ambulance (e.g. allow cars in front of the ambulance to move on through the lights), but that quicklt becomes a much more complex problem.
The laws on whether ambulances must stop at red lights and stop signs vary by locality.
It's almost always legally your mess if you don't stop at a traffic control device and are then involved in a collision — most laws and various insurance or service policy requirements tend to require "due care" or "prudent" or other such wording, and collisions are usually considered an adverse indication.
I've always wanted the traffic signal in front of me to be green on approach.
This reduces the numbers of stopped cars in my path.
Some of the drivers of the stopped cars at intersections will attempt or will perform odd or normally-unexpected maneuvers, sometimes leading to collisions. But mostly the herd in front of me scatters short distances, and generally resulting in intersection gridlock.
It's common practice in some areas to avoid using the siren in certain situations, such as when passing to the right of the bulk of (stopped) traffic waiting on a red, using the breakdown or turn lane at an intersection, for instance. Some folks will hear the siren and immediately block that lane, or they might turn into and collide with me.
The local traffic preemption systems all include a flashing indicator — a rotating red beacon on a cross-arm or a pole, for instance — indicating that the preemption request has been accepted and is occurring. This tells me that the other lights are all red. Or it tells me that I'm approaching a non-preempted light, and that the green in front of me might go red.
If the traffic lights are red and there's no preemption and no open and no bypass lane, then some of the folks at the front of the herd will inevitably not break that red, meaning local traffic gridlock, meaning that the ambulance waits (with the siren off) for the light to change.
Most of the time it's as simple as everyone pulling to the right and letting the ambo through the middle, but there's many situations where it just isn't obvious what everyone should do.
It would be great if there was an illustrated guide out there somewhere. I may have to look around.
When you're driving you either pull over and slow down (if it's safe to do so); or you keep driving at a safe speed and pull over when it's safe to do so.
If you're at a traffic light you can either try to move to the side without crossing the line; or you can stay where you are and allow the emergency vehicle to sort it out; or you can cross the line if a police officer directs you to do so (and blue lights is not a direction).
If you're at a red light with an emergency vehicle behind you, the best course of action might be to make a (safe!) right on red and then go around the block to get back on your original course.