Bubbles are mainly an aesthetic thing to put in a tank. They do cause oxygenation of the water at the surface by agitation, but they also cause carbon dioxide to diffuse back into the air, so they are not a good idea for planted tanks.
On the aesthetic side, though, they are great. My fish used to love them before i went planted... would swim in the current, let the bubbles carry them to the surface and then swim down, etc.
From a practical standpoint, they don't do much, other than use electricity, but they are really cool to look at, especially if you have those little l.e.d. lights that they sell to put in your tank now.... Place the stone either under the gravel, or into an ornament, such as an empty terra cotta pot, because, quite frankly, the little blue or green bubble stone by itself is ugly

... I had a friend that had the tubing run into the holes in the bottoms of a pot, a stone attached on the inside to hold the tube in, and fake plants, so that it looked like the bubbles were coming from the plant. it was pretty cool looking.
Sorry... rambling....