Given that we expect the Rebellion to continue to play a key role in the fight against the First Order in Episode VIII and Episode IX (duh), the woman calling all of the shots and making the critical decisions for the group is just a bit important.
To start with the broad strokes, Princess Leia Organa is now referred to as General Leia Organa because by the time we catch up with her in Star Wars: The Force Awakens she has become the leader of the entire Rebellion.
This led her to rise in the ranks, and by the time that 30 years had passed after the Battle of Endor, she was put in charge.
As we learn from her conversation with Han Solo, she could never fully leave the fight against totalitarian forces behind her, and returned to the world she knows best when her family fell apart.