The Raychi is mobile and you can explode them to spawn Algae on unowned ocean tiles, making it possible to create land bridges across any body of water. In fact, exploding Raychis to spawn Algae in enemy territory is usually the only viable way to capture enemy cities across the ocean, as doing so allows your Centipedes and Doomuxes to reach there.
However, the Raychi is not incredibly powerful as compared to, say, battleships. If exploding a Raychi can damage 2 or more enemy units, then don't hesitate to do so, as the Raychi has only 2 defense and can hardly survive till the next turn to attack again. This also helps with your Battle Skills score if you are playing Domination, as explosion of your units does not count toward casualties.