A light plane is a original unit from the game, The Conquerors. It is not used often in wars.
A light plane at the beginning of the game is used for getting to hard to reach places that take time to get to, like a island. When in the middle of the game, most people switch to heavy planes since it does more damage than a light plane.
A possible attacking strategy to use light planes with is for easy destruction of plants. If located on a map which includes water, this idea can go well due to the difficulty of reaching other energy crystals / oil.
When in a battle, the planes should have either tanks or soldiers to destroy any big threat to them such as anti air soldiers who specialize in destroying plane units.
Light planes can be used to bypass enemy defenses such as those ones may make on the map Fantasy 2. Since light planes have the same health as heavy planes and move faster, they are better at getting through a cluster of defense buildings and reach an energy crystal to build a base though they may not have as much firepower in attacking enemy units.