@frank26080115 A separate DAC gives me the ability to create and send waveforms directly from the MCU as well. If I decide to implement something like a low frequency arb. wave generator then it will be easier to send the data directly to the DAC rather than packing it up into a format that the decoder will understand just for it to be decoded again.
VLSI Solution brand MP3 decoders can also play WAV files, and WAV files are just a series of amplitudes. It'll work kind of like a normal DAC when you tell it to play a WAV file.
edit: but it won't work quite like an arbitrary function generator though, so your point is still valid