The other two bits are reserved as a kind of digital headroom. During processing, quantization noise initially soft becomes more perceptible. Digital post-processing generates rounding errors that are always apparent in the lowest bit. For these reasons, it's a good idea to work with 24 bits and convert the signal down to the 16-bit CD format as the final step in the processing chain. The great advantage here is that the potential of the CD format is exploited fully, right down to the last bit.
Tracks thus end up with the best possible dynamics, definition and fidelity.
Shannon's sampling theorem states that the highest frequency contained in the audio signal may be equal to no more than half of the sampling rate. Therefore, a maximum of 22.05 kHz can be represented by the CD standard of 44.1 kHz. With a sampling rate of 96 kHz, the audio spectrum can comprise frequencies up to 48 kHz. Peripheral noise introduced by audio clean-up operations can be shifted well out of the range of human hearing. Digital post-processing operations end up sounding much cleaner and more neutral.