iPhone 7+ home button only pushes down when phone is on?

My friend has an iPhone 7+ and we have been trying to figure out the reasoning behind the fact that her home button doesn’t press down when she pushes it while the phone is OFF, but pushes down while it’s on. We see no reason why the physical hardware is affected by the power down of the phone, because even if you pushed on it it wouldn’t do anything? It’s off? But when it’s on it works fine? It’s not an inconvenience or really a problem because she doesn’t need to use the button while it’s off, we’re just extremely curious as to why Apple decided to design it like this and we need answers or we’ll actually go crazy. We need to learn why it does this. It confuses us so much.