New answers tagged piece-information
The mask in the first picture is from the Hero Factory set Witch Doctor. The other pic looks like it's from the same set.
This, at least as far as my experience goes, is used simply to create extra friction in joints; to make something poseable yet stiff and able to hold its position. Sorry no pic, but a good example is the 8996 Scorpio XV1, a walking machine that uses these to keep the legs stationary unless you actually want to move them.
I don't find them very useful for storing bricks, but have used them as interim storage during a massive sorting project. Also used 2 to grow tomatoes last year, but they tipped over frequently. Best use I've found is as a small wastepaper basket. (I recycle most paper, but I don't know what else to call a small trash receptacle.)
According to LDraw, the slot is 8 LDU wide (1 plate, 3.2mm) and 5 LDU deep (quarter module or stud, 2.0mm). Here's the relevant quads from the LDraw file for reference:
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