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Other than being a standardized connector instead of a uselessly proprietary one....
Note uselessly: it allows Apple to charge a license fee to people who want to make accessories using the port. And it means that those accessories for your iPhone won't work with other manufacturers' phones, making switching more hassle.

That's why even though it would make sense otherwise to standardise on USB-C across the ranges, I still wonder whether they will actually do it.
 
It will be a short term pain, but a good long term change. I'm really happy they are taking advantage of it for connectivity to devices including external displays. This is a very welcomed and unexpected move by Apple ‘Mr. Proprietary'.
 
I have seen where someone jerry-rigged to do it on the old ones. I do have a WD Wireless Pro that I need to get replaced - it does not hold a charge for very long.
 
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