A preamplifier in very basic terms is a volume control (though it's not just a simple knob and such). The foremost reasons for hooking up speakers to preamp outputs on a headphone amp or in other cases on the DAC is so that there is a single chain for the audio (ie they'll use the same source/DAC) splitting only where necessary, as well as for a more conveniently-placed volume control.
For example, if you have a speaker system that has an existing DAC and power amplifier, you can replace the preamp in that system with the Asgard2, which also powers headphones if you need to use these for late night listening, while also controlling the volume for the speakers, and it uses the same DAC. If you have for example active monitors which have their own volume control knobs, on professional monitors you have one on the back of each speaker, and on consumer-oriented master-slave monitors (all amp circuits on one speaker), you still have its volume control on the speaker cabinet and even if it's on the front may still be too far from you - in either of these cases the Asgard can take the signal out of the same DAC then send it through its headphone amp circuit, or just pass it through its potentiometer then out its preamp outputs so you can control the monitors' volume.
As for using them with those speakers, check first with Schiit. Those speakers use a 3.5mm plug with a shared ground - most 2xRCA don't have issues with that but I'm more inclined to wait for a response from the manufacturer just to be sure (2xRCA taking an input signal from a 3.5mm output is the one that's sure to not have any issues). In any case, with the proper adapter (and assuming it won't have issues with the shared ground) you can control the volume on those speakers using the Asgard, however it won't be as great an improvement in convenience compared to how much easier it makes using active monitors, given those speakers already have their own volume control at a more convenient location (rather, that if the controls are on one speaker, you don't usually space them as much as much larger active monitors).