Why would they need to be able to dim street lights?

For use with wireless controls, the lights can be dimmed overnight when traffic is light (like between 1AM and 5AM for example) and the full output of the light isn't needed. Since these things are dim enough at just 20W I can't justify dimming them but on big urban streets that are brightly lit to protect pedestrians and heavy early-night traffic, dimming can be a good feature in the late hours of night when there's far fewer vehicles and even fewer pedestrians out. Freeways are another place where dimming can be considered. But for a residential area I wouldn't dim the lights. Most residential areas are already minimally lit.