Why A direct digital synthesizer builds a waveform numerically: a phase accumulator steps through a sine lookup table and feeds a DAC, all clocked by a single crystal reference. Because the output frequency is just a fixed ratio of that clock, set by the tuning word, the accuracy and drift of the output are tied directly to the crystal. That gives you a continuously tunable VFO whose stability matches a crystal oscillator, which is exactly what a modern transceiver needs.
Watch out The choice about a pure sine wave is tempting, but a DDS output is a stair-stepped approximation containing DAC quantization noise and spurious products, so it needs low-pass filtering to clean it up.
DDS = crystal clock in, any frequency out. Crystal stability with VFO flexibility, but spurs need filtering.