To save battery life, but we do not know how well the battery life with it on is with the new iPhone, plus if you do not have LTE in your area no since in having it on anyway.
Doesn't the specs say that it's 10hr (or 8hr I'm too lazy to look it up) for both 3G and LTE surfing?
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I still do not get it.
If I have to download 350k of data, no matter what it is, I will be charged for 350k of data. Could be LTE, GSM,2g 3g edge or whatever you want to call it.
If you need to watch your data, then watch what programs you are running.
Lets say you enjoy just browsing the internet for stuff to look at... aimlessly with no purpose, just to kill time.
Normally you go to Website A and then B, C, D etc.
But normally Website A takes 30 seconds to load, but now with LTE it takes 15 seconds to load so you get there 15 seconds faster and therefore you're onto Website B faster and then C, D and then maybe E,F,G and H. The amount of data you've consumed on ABCD is the same but now you've added pages EFGH to it as well, essentially doubling your data in half the time it would have taken you to previously.