Culled from the best recruits in the galaxy, field commanders were trained at one of the Imperial military academies located throughout the Core Worlds. Or in the Rebellion's case, they were trained at one of the Imperial academies and later defected or came from elsewhere and were promoted. They improved the morale of troops under their command. They were only lightly armored, with a helmet and vest for protection.
One Imperial field commander was sent in to Thyferra to find the rogue Moff Kalast. Due to the officers incompetence at ensuring a stealthy landing, however, Kalast escaped. As punishment for his failure, Darth Vader executed the commander.
At least seventeen more Imperial commanders were present during the demonstration of the AT-AT by Colonel Maximillian Veers. As a result of an invasion by Rebel forces, the commanders engaged in combat.
Another was in charge of the defense of a Tibanna gas refinery during a raid. Due to his failure to defeat the invaders, however, he was executed by Darth Vader.
Tyber Zann bribed a Field Commander to let him in to Palpatine's Sith Musuem. The officer said he would be in major trouble if his superiors figure out his role in the theft. Tyber, however, said he wouldn't have to worry. Zann then shot the officer, killing him.
In the game, Star Wars: Empire at War, Field Commanders increase the health and sight range of all nearby units. This could be explained as the presence of the commander increasing morale.