They are Entertaining
In most aquariums across North America, all you will see are boring plastic pirates with bubblers and ceramic castles, maybe a few neon plastic plants. Snooze! Moss balls are a live living plant that actually work in the tank and live in the tank.
They will change size, they will go up and down, and they will even move around. They are growing, adapting, and cleaning your tank and its water. Few people have them and many people will be impressed that you have them. Want a great story to tell?
They are an Activity for your Fish and Aquatic Creatures
One of the most pleasing things to see in an aquarium is when the fish and other creatures are having a great time without all of the excitement of their natural environment. As a tank keeper I am always trying to find ways to make things more natural in my tanks. I hate artificial layouts and dull and unhappy sea life.
Adding moss balls to the tank gives your creatures (in my case neon tetras, shrimps) something fun to play with. The tetras will brush against them for a massage. The snails will push them around as they place their suction all over them, and my shrimp will attempt to make games out of them though I am doubtful if they are winning their games. It's always entertaining.
They are a Natural Filter
Natural filters in enclosed and artificial environments are always helpful. They will reduce the frequency and expenses in changing out filters and water (though you should still be doing that). You should notice a cleaner water that is much less cloudy than before the moss balls were living in your tank.
They are Affordable
Moss balls are actually reasonably priced for what they do and where they come from. Many are shipped in from Asia and responsibly harvested from the wild. They are abundant in their natural habitats. To get a live, underwater plant delivered from so far at such reasonable prices is quite pleasing for this hobby.