> Is this needed because motherboards no longer include any floppy drive channels?

Yes, partially.

> I'm not yet grasping why Greaseweazle is better than a standard cheapy USB floppy drive.

1.) Support for non-standard formats.

2.) Ability to capture raw flux images, bypassing the limitations of actual floppy controllers, and allowing for weird formats to be preserved and written (ex. of flux images bypassing rare hardware restrictions: https://www.youtube.com/watch?v=vCU4xbHFb58&t=1538s)

3.) Can allow for drives with weird quirks.