More freedom? Like what?
Switch carriers any time you want, find some promotions from various different networks, not being locked in a long contract for years or have to pay an etf.
Makes more sense overseas, in the US there's many incompatible networks and very few carriers to pick from.
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If you buy the phone outright, what do you do when you want a new phone ? Go through the hassle of selling it ?
Yes.
Selling an iPhone is easy though. Not much of a hassle, it sells itself.