I don't have any, but I can see the sheer convenience of using a USB mic.

The factory equipped mic input in any computer will not have high quality D/A converters, and only allow use of dynamic mics.

I have older high quality sound cards, and 'semi pro' project studio quality USB audio interfaces
that supply phantom power to Condenser mics as required.

USB Condenser mics bypass the need for additional hardware,
which would suit many more casual users
who just want decent sound quality from easier to use plug and play products.