Why is my iphone turning off on its own?

Why does the iPhone/IPod Touch require ~20% charge before turning on?

• They remain usable until 3-7%, then power off. But once they die, you need to charge them to ~20%. Why the difference, if they can clearly run at the lower battery level? I figure this must be intentionally done by Apple, but why? What purpose does it serve?

• Answer:

  Maybe because there isn't a point in charging it to 5%, then having it promptly run out a few moments later? Also, letting a battery fully discharge frequently puts considerably more wear and tear on it. By charging it when it reaches just 50% (and doing a full cycle charge every once in a while), you can coax a lot more out of your battery. My iPhone turns on and has around 5-6% though and not 20% (or maybe I just never pay attention). Regardless, maybe Apple did this to prevent people letting their iPhones run out of charge so often and therefore avoiding additional wear and tear on the battery (since they are non-replaceable to the user).