The SOS, beacon and strobe functions on a flashlight are not something you often use and perhaps it can be said that they are seldom or never used at all by a majority of flashlight owners. But having said that, they are still useful functions when the necessity arises. As they say, “Why not have it and not need it, instead of not having it and needing it.” Like the flashlight itself, we get it because the need for it may arise and not because of having the need presenting itself and then getting the flashlight.
What are they...
SOS
SOS is a generally understood and adopted distress signal. It is taken from Morse Code where S is represented by 3 dots and O by three dashes and when keyed in the telegraph machine in series (...---...) it spells SOS, which signifies distress as first adopted by German radio regulations on April 1, 1905. It was set as a worldwide standard at the second International Radiotelegraphic Convention on November 3, 1906 and it came into effect less than 2 years after on July 1, 1908. Since that time many other ways of alerting others to a distress situation have been developed and adopted and this includes light sources like a flashlight. The repeating pattern for SOS is three short bursts, three long bursts and then again with three short bursts of light, and it has been in use for a long time. But these days many flashlights have this SOS pattern built-in and require no manual on-and-off manipulation of the flashlight to get the pattern right.
It is hoped that one never has to use this function since if it is ever required it means something critical has happened and it is likely that life or death hangs in the balance. Faced with any form of distress, either man-made or by nature, you can rely on this function to alert others that you are in some sort of distress and need help.
Beacon
Beacons are designed to attract the attention of people to a particular location and signal them about something. The interpretations of the signal can be varied but the basic idea of the attention it provides is universal and they have been around since ancient times. Today, even flashlights are equipped with this function and they also provide portability that other beacons do not have. In the beacon mode, a flashlight will emit long bursts of light in slow repeating intervals.
As a means of navigation, a beacon can be set as a point of reference when you or others in a group become separated or lost. It can also be a way to alert others as a signal for something with a preset meaning. In ancient times, fires that could be seen for large distances were set to alert of threats.
Strobe
Strobe lights have been around since the 1930s and they have been used in various applications. Some are used to stop the appearance of motion and in flashlights their use has been centered on a disorienting effect of flashing lights that was first noticed in the 1950s called Flicker vertigo or the Bucha effect.
Strobe function in a flashlight is primarily used as a defensive measure against threats and secondarily as a light signal. The disorienting effects, night vision disruption, temporary blindness (depending on the light intensity) and possible fear of the shock of getting exposed to the fast cycling of the light going on and off for the strobe light all contribute to allow for a window of opportunity to either disable the attacker or make a run for it.
All of these extra flashlight functions are there to increase your chances of survival and improve your effectiveness in various situations where light is used. The need to use the SOS, beacon and strobe functions on a flashlight might not seem useful at present because the situation when they are required is not there yet but if or when it does you'd better have them or you might regret it.