Coincidentally, I just used one in a Stackoverflow answer here so I could treat a word that was made up of 6 bit fields as two 16 bit unsigned integers.
Years ago, I also used one for (the first) ARM C compiler - the instructions in those days were all 32 bit, but had different layouts depending on the exact instruction. So I had a union to represent an ARM instruction, containing a set of structs which each had the appropriate bitfields for a specific instruction type.
