So what do you use this for? Playing audio back in a large room like a restaurant?

With this you can cheaply replicate a Sonos style whole home networked music system, by having the recievers be something like some raspberry pi zeros wired up to some amplified speakers.

Ive been wary of stuff like Sonos due to the cost and the proprietary nature of the tech, but this Scream driver seems to be a nice simple alternative.

I don't have a sonos but isn't one of the selling points the fact that clients are kept in sync? Does this actually do that? Shairport-sync does.

Well, if the receivers are always-on, and listening for the UDP packets from the Windows PC, then surely any lag causing de-syncs would be minimal - in the millisecond range.

For a one-speaker-per-room setup, it would probably be 'good enough'.

Needs investigation I think.