While not a survival essential, there can be survival uses for cameras (or at least their flashes). The most interesting story I recall was a German tourist who got lost on the ice of the North Sea and fired off his flash hoping someone would see him & help out. It turns out a woman watching a webcam for the area saw the flashes and contacted police who ended up saving the man.

Here's another case of a lost hiker using a flash as a signalling tool: Lost hiker uses camera flash to signal location