Soundcard itself is a DAC. Why people using external DAC is to get better audio, since the uses of switching PSU makes it noisy, and I don't mean fan noise

If you uses standard multimedia PC speaker, it wasn't necessary. Any good soundacrd will do, even modern onboard sound should suffice. Those brand you mention is just standard multimedia PC speaker, only AegoM is more suitable for music listening

The USB connection is just mere connection to the PC, as it allows the PC to 'see' that it connected and transfering digital audio data, not supplying power

A good soundcard like Audiotrak Prodigy HD2 would suffice for music