[oneshtein]

General Relativity is a mathematical model. Model needs mapping between physical world and model.

[ben_w]

My phone's GPS only works because that mapping keeps being better than anything else we've tested on that scale.

The atomic clocks (as I mentioned in another comment) demonstrating gravitational time dilation within a building is another fun example.

[oneshtein]

Your phone works because engineers did it.

I see strong annual signal in GPS: https://link.springer.com/article/10.1007/s10291-017-0686-6 .

Why it there?

[ben_w]

> Your phone works because engineers did it.

The engineers implemented results from GR.

> Why it there?

Not that it matters, but the answer is in the introduction.