I know the ones on the side of roads have them so they are more visible to traffic at night.This is the case in most places where there aren't many street lights. In Britain we have cats eyes in the middle of the road.
Thanks folks - thought it might be something to do with visibility... although some of the ones I sawwere nowhere near roads - perhaps they were there to guide people staggering home after imbibing a drop too much Robola!
Bob - Cephalonia is one of the Greek Islands - go there if you can, its gr8!