Why is it that in Apple software (OS X and iOS,) the "Inverted Colors" display feature is considered an accessibility option?

I understand that some users are color-blind. This would justify the Black & White, or grey-scale modes. What I don't understand is how or why does inverting the display color help someone with any specific, visual impairment or dysfunction.

As a programmer that wants to understand the need so that I can develop better, accessible software, what purpose does this feature serve to the end user who has some form of visual impairment?

NOTE: I felt that this was a hard question to categorize on StackExchange. I settled here on Programmers because I assume that questions of accessibility are important to all developers and this question sits somewhere in the middle between topics that StackOverflow and SuperUser may cover.

Also, this question isn't specific to Apple software. I've just noticed that this feature has been available on Macs for a very long time, it's a feature on iOS, and it's always associated with the Accessibility settings.

If I can garner some information regarding the needs of some users, I think that I can develop better, accessible software.