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If you mean this message:

iphone-dead.jpg


Probably to prevent you from using it with 1% left and running it down again.
 
Because the iPhone draws like 3% battery when it turns on and its a failsafe incase it is unplugged when booting.
 
oh please, stop being ass hats guys. not always are you near a charger. when your on a long phone call your not eyeing the battery
 
It's to charge up to 4%. Safety measure to prevent software corruption by forcing the phone to shut down with too little battery power remaining.
 
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