Green Man Authority

The design of light-controlled crossings almost universally communicates to drivers that they have a default priority over any pedestrian interest in crossing the road. For as far as line of sight is available there is a green light indicating that they can travel up to as close to the speed limit as other traffic and road furniture allows. This naturally creates a hierarchy that tells people in cars that they are more important in that space than people not in cars. This in turn contributes to the attractiveness of private vehicles as a mode of transport. Light controlled crossings tell pedestrians that they are expected to wait on a pre-defined and limited inconvenience of motorists.

This change in operation of the lights inverts that priority. Pedestrians are always told that they can cross without waiting unless told otherwise. In the meantime, motorists approaching the crossing are told by a red light that they are expected to slow down and wait, like pedestrians usually are, until they are given permission to proceed.

You're right that they probably are of limited effectiveness in quiet spots, but then quiet spots are more likely to be served by zebra crossings anyway. But traffic flow anywhere already varies according to time of day and demand for local facilities. So even in a busy spot there will be times where traffic flow diminishes and the green man authority comes to the fore and puts pedestrians first.

For some people, a road that isn't that busy is one that will be crossed regardless of whether there's a green man or a red man showing. You might not even push the button, just wait for an appropriate gap in the traffic and walk across. But for others, perhaps less able-bodied or encumbered by a pram, they're a lot more likely to wait for an indication that motor traffic is being told to wait for them. Something like this makes that crossing space a lot more equal for them.