What benefit does a DDC really do for a DAC?

The general consensus is it really shines when wanting to turn a USB feed into a I2s connection to the dac. It does wonders for me. Innuos zenith mk3 USB to DDC U18 I2s to Audio Mirror Tubadour V. The veil being lifted saying is no exaggeration in my case. A huge difference from USB directly from streamer to dac. 

For sure, using a DDC will allow you to use a variety connections to your DAC. If your DAC sounds best from an input that is not available as an output from your streamer, then interfacing with a DDC that allows you to use that type of connection could make a tangible improvement.

Other than that, the resulting sound quality typically depends on the DDC and the DAC. Most of them are said to offer benefits of better clocking and lower jitter, but having used a Denefrips Hermes and Singxer SU-6, I cannot say that I heard any reliably consistent sonic improvement from either. However, I did appreaciate the multiple output options that gave me the ability to use whatever DAC input I wanted.

Using a DDC also allows you to use a dedicated master clock to synchronize the entire digital audio chain, including your DAC, which some report to significantly improve the sound of their digital playback.

I do not use a streaming service, and have no plans to add one. I have an extensive collection of SACD, DVD Audio, BLU RAY, and Japanese sourced disks in various formats. I love having physical media. I also have around 40 various Grateful Dead disks as I was a Deadhead from 89 to 95 with 27 shows of Jerry and company. I don't play music everyday, as I treat the system like a Ferrari, or rare muscle car. For special treat for myself and others.