Ramp metering (wiki) seems to be a common practice at freeway entrances in Wisconsin. The concept is simple: by allowing vehicles onto the highway in small numbers, you can reduce traffic up ahead as well as improve safety by reducing the friction caused by merging traffic. In the event of an emergency, holding traffic back away from the scene reduces the number of interfering cars that first responders have to deal with.
Street metering could be very useful in cities with limited space. Since traffic engineers primarily concern themselves with traffic flow, one thing they focus on is storage: space for vehicles to wait during a red signal phase or for turns. If you metered the traffic coming into a busy area, you wouldn't need as much storage in areas without excess room.Those of us who always hear the tired lie that "there's no room" for bike or transit priority -- there is always room unless you give car priority -- might think about how metering could reduce the storage engineers say they "need" in specific locations. So you can have your protected bike lane in the narrow road section as long as not as many cars are allowed to queue there.
As a transit priority measure, metering could be used in advance of a bottleneck such as a bridge or business district, allowing buses to pass stopped cars in a bus lane or shoulder and move in front where the street is clear. Think of it as a queue jump where traffic is held for a minute or two whenever the bus is detected as approaching. A meter could also be used at an intersection with a transit corridor so that buses and streetcars don't deal with as much traffic.
Do you know any current examples of metering on city streets?