Driftwood in the aquarium provides a haven for fish and invertebrates, food for certain species such as plecos, and offers natural water conditioning. Most of all driftwood looks nice.
Madagascar driftwood sinks but needs several days to soak, just place in the aquarium and eventually the driftwood will fall to the bottom. To speed the process along tie a rock and sink the wood. And of course the pieces are selected to fit within a live fish box.