Metal Slug is probably one of the most popular IPs in the history of arcade gaming
When the Parachute Truck was first released, it dropped a hail of projectiles at a good range that did penetrating damage as long as they touched an opponent, decimating even the biggest units in seconds. It got so bad that the following update nerfed them greatly, giving them less range, giving them a longer time to build, and making their attacks no longer penetrate.
The importance of knowing your troops will multiply your chances of success on the battlefield. Taking into account unit differences like health and action points, standard and special attacks, movement and production speed will factor into what units you deploy for offensive and defensive situations. The strategy comes into play by utilizing various units to create a winning offense to take out the enemy base. A good example of unit knowledge comes into play when you have two units with similar special attacks like Tarmicle Roving III and Fiolina Germi, both use a spread attack to dispatch enemy troops. The difference is that Tarma has a lower AP cost and a higher HP. He offers a faster movement and production speed, which makes it easier for you to quickly dispatch enemies. Fio has a lower HP, movement and production speed, but compensates with power and special attack spread. This makes sense, since a Rocket Launcher would have more reach and power than a shotgun. Knowing these differences in unit abilities will pay dividends on the battlefield, as each situation calls for a particular unit.