Raina and Bluntschli's meeting is important because it sets the plot of the play in motion. Even more importantly, it creates a conflict for Raina and causes her to have to rethink her ideas about what constitutes a hero.
Sergius is Raina's romantic ideal of the brave soldier. Bluntschli, a mercenary and a deserter who escapes to her bedroom, is everything she knows she should despise. Nevertheless, Bluntschli, with his frank talk and honest action, is the man who most deeply attracts her. Once Bluntschli crosses her path, she is forced to reevaluate her relationship with Sergius.
Bluntschli tells Raina that Sergius is not a hero and only won his battle because the other side was even more incompetent than he was. Bluntschli challenges Raina's notions that war is glorious.
Once Raina starts to get to know Bluntschli, she can't help but become aware that her romance with Sergius is based on an illusion. Bluntschli's arrival is providential, because it saves her from a marriage with a man who prefers her maid and throws her into the "arms" of the man she truly loves.