For certain individuals that are visually impaired, inverting the colors of the iPhone and iPad screen may make it easier to read, particularly for users who are colorblind or partially colorblind. This is actually a built in accessibility option for all iPhones and iPads. So whether you yourself are visually impaired or you know someone who is, it could make reading text and viewing content within iOS much easier.
You'll notice the screen colors will now invert and somewhat resemble a photo negative. This can make it easier for those with certain visual impairments to pick out content and navigate easier. You can pop back into Settings at any time to disable it if you'd like to later on.