Why?
Because it’s better designed, aesthetically and functionally. It shows a battery icon in iOS and the silicone material feels premium compared to cheaper cases. Since it’s a hump that doesn’t consume the entire backside of your phone, it’s also probably lighter too. It also supports wireless charging and of course it has the Apple logo, which some will laugh at, but its a touch that I can certainly appreciate.
For those that don’t care about these “niceties”, by all means go for the cheaper alternatives and be happy, but personally I would still pay more for the Apple battery case myself because I do care about those little things.