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DAC in general, is used to bring your I,Q baseband or IF digital information to the analog domain to be mixed and modulated to proper RF in order to transmit through an antenna.
DAC as Low-pass filter? I guess you are thinking about the x/sinx performance of the DAC in frequency. Basically, DAC fucntionality is equivalent to locating a rectangular analog pulse with the height of your digital data for each DAC clock instance. This in math is equivalent to convolution of your digital data (as set of impulses with specified values) and a rectangular pulse in time domain. As you know, the frequency interpretation of the rectangular pulse is a sinc(x) = sinx/x . That is why in most cases, if you are working with wide-band BW, you need to compensate forthe droop of DAC using a x/sinx compensator before DAC.