People like the way they look. They are cladophora algae that grows in areas where the movement of the water constantly rolls them, so they become ball-shaped.
I will never use them again. I had a few in my tank, and the cladophora started spreading over my driftwood, plants, and substrate. As mentioned, nothing eats it, and I haven't found anything thats killed it, so I haven't been able to eradicate it from my tank yet.