The mic and speakers on the laptop are designed for a single listener/speaker sitting directly in front of the machine. A USB speakerphone is designed for mul

The mic and speakers on the laptop are designed for a single listener/speaker sitting directly in front of the machine. A USB speakerphone is designed for multiple speakers and a wider area in which they could be positioned. You also get some advantages from isolating the mic/speaker from the laptop chassis, noise control and such. Same reasoning as using an external DAC/Amp for headphones rather than the onboard interfaces.