@staticV3 is right, when your config is suffering from the ground loop noise then USB isolator can solve it, or you can also solve it using the Optical Toslink. This idea is to disconnect the ground of active devices.
In this review the noise of the DAC without USB isolator is low, you are correct, but it is so in this given case and even though USB isolators demonstrated their ability to eliminate the added noise by disconnecting the grounds of 2 active AC-powered devices - PC and DAC itself.
The amount of noise depends on the configuration of the system because all systems are different and have different components with different side effects.
For example, in my config I can easily hear noise (like modem communication) when something happens over USB bus, or just periodic ticks, also low-frequency hum. In my system ground loop is created via the external speakers which have 2 line-ins: one line-in is connected to PC's sound card, another line-in to my DAP which in its turn is connected to PC via USB. Even at low volume I can hear annoying noise. Connecting DAP via USB isolator disconnected grounds and solved noise problem completely - I get absolute silence even if I set amp of the speakers to its max amplification.