One of the main advantages of DDS is that the frequency accuracy of the output signal can reach the level of the crystal controlled oscillator used as the reference signal of the generator. If you want to achieve higher accuracy, you can also use the function generator itself to produce temperature compensated crystal oscillator. These signals can provide frequency accuracy above 0.1PPM. In many cases, the function generator can also lock the frequency to an external laboratory frequency reference source to generate a signal with ultra-high precision.
The second advantage of DDS is related to the first: a DDS generator can generate very high frequency accuracy. The digital circuit of DDS signal generator can achieve the same frequency accuracy as the digital circuit. If the DDS circuit has a 48-bit counter, it can provide up to 48-bit frequency resolution, and recently some DDS synthesizers have used counters with more bits and higher resolution.
The third advantage of DDS is that if you have RAM waveform memory, the DDS function generator can reproduce almost any waveform. The DDS signal generator operates by playing the waveforms stored in memory. If the memory is read-only, only the waveforms stored in the ROM can be generated. In general, sine, square, triangular, and similar waveforms are built into each function generator. But for square waves with a duty cycle of 10%, or triangular waves with symmetry of 58% (instead of 50%), a DDS generator with read-only memory alone is not possible. If the DDS circuit has RAM for storing waveforms, the controller circuit can write any waveform to RAM and replay the waveform through the synthesizer.