Do I really Need a Digital to Digital Converter?

Not really.

At most, a USB isolator may help in some situations when galvanic isolation isn’t built into the DAC, but what I’ve seen is that you may make things sound different by exchanging one set of jitter for another. That is, you can see the jitter signature change, but rarely is it "better."

DACs made in the last 10 years have fabulous jitter elimination built in and USB asynchronous is usually the best. Also, old DACs performed remarkably better with high frequency signals than they do now. That was another reason to consider an extra external devce, like a sample rate converter like the Wireworld Remedy. Works wonders on old DACs but not really helpful anymore.

The only real use I can imagine with a modern DAC is if your DAC lacks asynchronous USB or streaming. Another possible benefit would be galvanic isolation which, ODDLY, not all DACs have built-in, like these:

https://amzn.to/46ZhIKO

USB isolation can be really helpful when streaming directly from a PC.

I think that you can only benefit if the company making it does it better than the company making your DAC/Streamer.  

So in your case, I'd say no.

All they really do is expand input capabilities to your DAC from a USB. Other inputs might sound better (or worse.)

Might not.

I had an Ares II fed by an Iris and when I added the Iris, there was a noticeable improvement .

*note - my Ares II didn’t even have I2S and it was still and improvement.

Also, when using my 30 year old California Audio Labs Sigma II tube dac, the Iris helped it in a few ways.  (1) It stopped the relay from triggering when skipping tracks while I’m streaming and (2) more importantly , it just brought more clarity and detail in the sound.  

All that said, I sold both the Ares II and the Iris and now I’m just running coax Spdif into a Border Patrol dac from my streamer and I’m happy as a clam.

@j_andrews

... would you share which streamer it is, and if you stream high res audio >44,1 / 48 kHz? 
Cheers!