During World War II, hundreds of air raid sirens were installed throughout Los Angeles County as part of a civil defense system. The idea was to warn citizens and mobilize local defense plans in case of an enemy attack. The sirens were used in 1942 in the so-called "Battle of Los Angeles", an apparent false alarm that caused panic and injury, resulting in the deaths of five people. The sirens were also set off as part of the celebration on August 14, 1945, the day that Japan surrendered, ending the war.
After the war, the sirens were switched off, but during the Cold War of the 1950's they were reconditioned and reactivated, and some new sirens were added. They were tested on the last Friday of each month, which probably would have been a good time to attack the city, if one were so inclined. They also occasionally frightened citizens by short-circuiting at odd hours. Over time, the sirens deteriorated, replacement parts could not be found, and the system came to be seen as useless. They were officially silenced in 1985.
Because of the potential expense, there has been no project to take down the old sirens. From time to time, they are removed as safety hazards, or to make way for new projects. Most people don't notice them, just a few more poles in a city full of light posts, electrical wiring, telephone poles, satellite dishes and cell towers. Dennis Hanley, whose website documents the location of these relics, reports that as recently as April 2007, more than 85% of the sirens in the Valley were still standing, compared to nearly 77% city-wide. You can see Dennis's Air Raid Siren Page for a detailed list of locations.