What is the purpose of CRC_IDR in STM32 processors?
According to their reference book


  Bits 31:8 Reserved, must be kept at reset value. 
  
  Bits 7:0
  General-purpose 8-bit data register bits Can be used as a temporary
  storage location for one byte. This register is not affected by CRC
  resets generated by the RESET bit in the CRC_CR register.


I would guess they had plans when making the CRC in silicon to use 4 bytes and in the end only needed the upper 3.