If you already own a DAC that has a good clock, galvanic isolation and all the I/O ports you want, you don’t really need a DDC.
@devinplombier My experience along with many others here is a bit different. Adding a DDC can provide a better signal to a DAC, and even a DAC with a good clock benefits and sounds better by having to do less work to clean up the incoming signal. Further, if your DAC accepts i2S and the DDC allows you to take advantage of that input the DAC is also freed from having to separate the clock from the data signal that can also provide significant sonic benefits. I’d bet there are very few DACs that wouldn’t benefit in one way or another from a good DDC and is likely even more true if you’re feeding a DAC from a non-optimized, multi-use computer. I’d encourage anyone with a DAC to at least try a DDC as it can be a relatively cost-effective way to provide very meaningful improvements. That was very much my experience anyway.