Honest question - what's your use case for needing GPS indoors? I generally know where I am when I'm indoors :)

Maybe indoors is the wrong term: as soon as you don't have direct sky visibility it's relatively hard to get a position.

Some examples: on a train, on the underground, in a train station, in a mall, in an office building, ...

You're in a large building you're unfamiliar with. Particularly one with an unusual layout, like a mall or hospital.

It's useful in shopping malls, airports, train stations, car parks and so on. Anywhere you need to navigate a large complex.

Not OP but navigating large malls, subway terminals, etc is nice