Dark mode is not a fad, it's simply a good option to have in the valid cases you describe[1], and has been available in many products for decades. The reasons

Dark mode is not a fad, it's simply a good option to have in the valid cases you describe[1], and has been available in many products for decades. The reasons we're seeing more of it are that developers are switching their bad hard-coded UIs to more flexible systems which allow it, and now that even MacOS/iOS supports it, there's an expectation for more and more apps to conform. [1]: another valid case is apps with a strong emphasis on graphics: Photoshop, YouTube, Instagram, etc. A dark UI mitigates visual interference with the content.