This is a corner connector/attachment piece for HSW storage. It allows you to “turn the corner”, i.e. connect 2 areas over a 90 degree angle.
On the first HSW plate, it would use the first available hole, on the second HSW plate, it would use the second available hole. At least 2 is needed top/bottom (different orientations - or the same for consistency) to connect 2 plates together.
2 types are provided - those with a hole through the center and those without - see the pictures for the various types. In both cases there are plates for printing multiples at a time, 2x, 4x or 8x.
There are other models on MakerWorld that requires glue and certainly has a nicer appearance - in my specific use I really wanted to have a connector hence this model was born.