Princess Leia was forced to wear this outfit after being captured by Jabba the Hutt. It didn't stop her from using his own chains against him, escaping his barge, and rescuing Han.
Leia Organa has five personas in the game, which are treated as different characters. Slave Leia was introduced in version 1.0.
This version of Leia does not unlock any scenes. As a Rebel, she is actively chased off of any Imperial level.