AD916x: NCO complex outputs and DAC output signal


I have a question from our customer about the NCO complex output data and DAC output signal.
The figure-172 indicate that the NCO output the  I-phase data(OUT_I) and the Q-phase data(OUT_Q).
This I-phase data consist from base band I data and Q data, so I think it is enough the I-phase data to generate DAC output signal.
What purpose the usage of the Q-phase data of the NCO output?

Best regards,