what is the purpose of the analog part in the DAC

Yes all DAC chips output analog signals but they're often not in the right format for sending to a preamp. For example many DAC chips output current rather than voltage so require an I/V (current-to-voltage) stage. Many DAC chips output balanced signals which need to be converted to single-ended. Then there's the question of ultrasonics which need to be removed. Finally there's buffering to drive a cable and potentially a poweramp. 

thanks!!

Yes all DAC chips output analog signals but they're often not in the right format for sending to a preamp.

Actually, this is incorrect.  DAC chips only output DC (direct current) pulses that are at different levels.  The "I/V stage" is an analog stage and it is required at a minimum.  The job of the I/V stage is to take those DC pulses as an input and help to try to form that into an analog waveform.  The byproduct of slow slew rate and voltage increase with this signal amplification stage is used to help that.

There can be other analog stages, like a low-pass filter stage that works to remove those ultrasonics.  An output buffer stage may also be added to help drive the input impedance of the target device (i.e. preamp or amp).