replace screen on phone

The home button itself is paired to the iPhone's motherboard. The home button contains patented technology by Apple that allows the device to read and register fingerprints. It needs to be transferred to a new screen so you can continue to use that feature. If you break the home button, after market home buttons are available but they don't contain the technology to read and register fingerprints, only go home. If you're asking why in terms of why it’s designed this way, it’s just an older Apple design. On newer iPhones, they moved away from this setup entirely, which avoids this issue.