A USB> SPDIF converter generates a SPDIF stream.
The rate of this stream is used by the receiver to generated the sample rate.
A clock is an analog device.
It has the normal analog imperfection
Its cycle to cycle time will vary, this is called intrinsic jitter.
The bigger the intrinsic jitter the more input jitter at the DAC.
This is the simple part, better jitter performance means less input jitter at the DAC.
However, DACs might employ all kind of tricks...