Sometimes there are roaming patrols of NPCs you want to not engage. Powerdown is the way to disappear from their radar until they leave.
I haven't had as much success with dropping aggro mid-combat in here as I did in live. Back then it was godly when it was maxed -- as long as there was no damage ticking on a mob or you weren't being damaged right that second (You couldn't use repulsor with it) you could shut down, let your reactor/shields recharge and then resume the fight. The NPC would heal a bit as well, but not at anything resembling your recovery. You could do this over and over, until said mob was dead. It just took a little time, but I killed a ton of critters I would have had no business fighting otherwise this way.