Everyone likes to be noticed. But it's especially good to be noticed by drivers when you are a pedestrian. Those that wear reflectors can be seen by drivers much more clearly from longer distances than those without. Most accidents involving pedestrians happen in builtup areas with street lighting because the driver din't see the person in time to stop. Therefore a reflector is equally beneficial in the city as it is in the countryside (Source NTF).
Glimmis reflectors are delivered in packs of 10 reflectors.