Why do I have to reset my old iPhone when I switched to my old Droid on Sprint?

So, I had a really fun time switching from my iPhone 5c back to my old Galaxy S2 Droid fr an upcoming trip overseas. Basically, I wanted to use the old Droid in case it was lost or stolen. Just before activation, I was given this message:

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"Reset your old Apple device

Important information about moving from an Apple device.

Print the following tutorial so you can reset your old device once your swap is complete.

After you have activated your new device and transferred your information, be sure to reset your old device. To receive the Sprint Buyback credit, you must remove your personal information."

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It then walked me through a tutorial explaining that I needed to turn my iPhone 5c back on and essentially find the "Reset iPhone" option in settings. Why is this necessary, and will I face network problems if I don't do this when I switch back to the iPhone?

Because iphones have security that makes sure only your password can access it. So they want you to wipe it first.

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