I think it's done to more easily deal with internationalization. Instead of making the design fit the translated text, just remove the text altogether and stick with just the icons.

Well "mystery mean navigation" refers to something specifically where there's little chance of understanding what the icons refer to without rolling over them, much like "mystery meat" in your fridge that you can't remember what it is without opening the tupperware.

For accessibility, they still need the text. So only hassle saved is the minor one of designing a bit larger to fit languages that need more pixels.