Yeah you probably don't need it in slow moving urban traffic but I saw a video with some bike packers in Australia who used Varia. They were on rural roads shared with road trains.

Road trains a very big, move very fast and they don't stop for anyone on a bike. So knowing they are coming up on you from a distance gives you time to get off the road.

That's a very special usage case but I think any rural road with fast moving traffic would also benefit from the early warning even with just cars to contend with.