In my yard, birds are in danger from house cats, hawks, snakes--and my dogs. It's a dangerous world.
The birds like a place where they can perch up above and look things over to see if any predators are lurking before they come down to the water. Sometimes there will be one lookout bird to sound a warning if needed. They also like some brush to hide in if needed, but with no real low branches to hide the cats.
Great question!
Not the best picture, but you can see why sometimes there's only one bird on my feeder.
