Does anyone recall why we have this?

I have data formats which are bit oriented and there are complex types which would naturally not be a multiple of 8 bytes long, e.g., 1 bit field, 3 bit field, 10 bit field, 6 bit field = 20 bits.

I can't think of any reason for this restriction other than to explain how fillByte is used to fill in unused bits. But I think we can say that any unused bits are filled in with bits from the fillByte, and we don't have to be specific about which bits from the fillByte.