What is SOS on iPhone for?

If your iPhone shows SOS where the signal bar used to be, your service area may be weak. If you can, try moving to an area with better service if you need. Without service, if your iPhone shows SOS in the service area that means your phone is able to make an emergency call, if needed. During this time, you are unable to use cellular data, send iMessages/SMS. If, you are asking about when you hold (power and volume on iPhone's without a home button, for iPhones with a home button, just the power button) and see the SOS slider there, that is for when you are in an emergency and may need to quickly call for SOS (First responders will respond to your location if this is used). If you were to slide that or keep holding down (If you keep holding down, an emergency sound will trigger to alert you that it is calling for SOS if you keep holding, this setting can be turned off by going into settings), your iPhone will alert your Emergency Contacts with your location, and the time the SOS went off, then your iPhone will connect you to an emergency line (Such as, 911, 111, etc) Hopefully this helps!