Does this really need an explanation?
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Non-replaceable batteries provide zero benefits.
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Swappable batteries allow you to just carry extras instead of being tethered to a charger. I used to literally never plug my phone in. Just swap batteries one or twice a day.
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It's the first component to die in most phones, and people can't be bothered or don't realize you can have them replaced, so they just throw them in the garbage. That's why they started doing it in the first place.