The importance of the external conflict that Holden experiences with various people during the course of the novel is that it increasingly highlights to him how much he needs help and needs to see a therapist. The novel is of course written in the first person, and as readers we are therefore presented with Holden's account of what he thinks is happening and his thoughts and feelings about those around him. However, it is clear at points that Holden, because of his emotional and psychological issues, is a rather unreliable narrator. For example, during his fight with Sally Hayes, she tells him to stop shouting, but Holden reflects that he wasn't. Given how upset she was and how overwraught Holden clearly was, it is obvious that Holden was shouting, but that he was not aware of it. Note how Chapter 17 ends, however, where Holden states Sally would have been terrible to run away with into the countryside:
The terrible part, though, is that I meant it when I asked her. That's the terrible part. I swear to God I'm a madman.
Holden receives a moment of insight at this point where he recognises he does have issues. Although this event in itself is not enough to make him see a therapist, it, combined with the other events he experiences, help to make him see he has a problem, which in turn encourages him to go and see a therapist. Holden during the course of the novel realises more and more that he is in a bad way and needs help.