The Gaia does a few things.
It can convert virtually any input type to virtually any output type, so you can optimize the connections to your devices based on what works best and what cables you have available.
It isolates the inputs from the outputs and removes a lot of the noise.
It has extremely high quality oven-controlled femto oscillators (comparable to what you'd find in the best streamers and dacs) so that the signal delivered to the DAC is as clean and jitter free as possible. It can also sync to an external clock if you use a master clock source, or if your DAC can output clocks (like my Denafrips Terminator Plus).
If purchased new, the Gaia is about $1700 (price adjusts based on exchange rate with Singapore dollars). The current model Bryston streamer (BDP-3) sells for $4100. Mine was the previous model, although used the same output board as the BDP-3. So as a Roon endpoint, it was equivalent.
My main motivation for purchasing the Gaia was to have a way to use i2s with my DAC (which at the time was the Denafrips Terminator - since upgraded to the Terminator Plus). I had read that i2s worked best with the DAC. I had not intended to sell the Bryston when I bought it, but I found that connecting directly to the Roon server via USB worked at least as good (I think maybe better, but the difference is very subtle).
Using i2s from the Gaia to the Terminator made a very noticeable difference in sound quality. I haven't had a chance to compare other inputs on the Terminator Plus (I've only had the new DAC for a few weeks).
I'm using fairly nice cables for the USB and i2s connections (Audioquest Diamond USB and Tubulus Argentus HDMI/i2s). But I initially connected the Gaia to the Terminator using a cheap generic HDMI cable ($12 from Amazon) and even with this cable, the difference was significant compared to using other inputs connections.