Why is Septimus here?

At first, I wasn't sure why Septimus was in the book. He seemed like an out of place guy who didn't fit into the upper class society that Clarissa and the book seemed to be oriented around. In the passage where we first meet Septimus, his reaction is so radically different from everyone else's when the car backfires that it's really quite shocking to read. After reading the whole book, Septimus's character just seemed more and more interesting and I really wanted to find out why Virginia Woolf wanted to add in Septimus.
I'll admit that I was disappointed when I scanned through; I didn't find anything that really stood out. When comparing Septimus to other characters in the book, he seems, at first glance, to be opposite of Clarissa's. Especially after the panel presentations, I saw in rather vivid detail how contrasting Septimus and Clarissa actually were. I decided to make a thorough comparison of both Clarissa and Septimus and see how similar they actually are and also perhaps realize why Septimus was placed into the book. For one, Septimus is submerged in his own world, distrusting of everyone. Whereas Clarissa seems to be an outstanding citizens, throwing parties for her husband and is able to go out without freaking out to events that happen around her.
However, we know that there must be more to both characters. In class, it was mentioned that in the original version of Mrs. Dalloway, Clarissa was meant to commit suicide. This seems very similar to Septimus. In fact, even though Clarissa didn't commit suicide, they still have a lot in common. Throughout the book, Clarissa is constantly reminiscing about her life, although in private. She feels as if she has aged and at the end, she identifies herself with Septimus.
In fact, there is more to be identified about the similarities between Septimus and Clarissa. It is rather obvious that Clarissa is extremely conscious about the judgement of others and the way the world perceives her. She feels that the only time she can be herself is when she is alone and she frets about her looks, how she is aging. Septimus, despite being in a different world, is also conscious of the way others look at him. In a particularly memorable passage where the car backfires, he worries that he is in the way of others as everyone seemed to stare at him. This seems different than the initial view that we get of Septimus, who seems off in his own world, paranoid and not truly understanding what is occurring around him when in fact he is worried about other people's opinion of him, though he might know that they are judging him because he's acting so strangely.
There are also a large amount of differences between Clarissa and Septimus. Clarissa seems to be surrounded by life; she throws parties, she enjoys flowers, she seems like a typical citizen. Septimus has a very different take on life. He sees everyone as wearing masks and hiding their true selves and ultimately, he committed suicide because it seemed like he didn't want to continue to interact with those people, in particular Bradshaw and Holmes. However, Clarissa might not be as ordinary as she first appears. We know that she has been thinking about life and death throughout the book and at the end, she remembers incidents from her past in that dark room that she feels was the moment that she could die and be completely happy (this is the time at Bourton where she was with Sally and Peter).
I think that Virginia Woolf introduced Septimus into the novel because she wanted to use him to further explore the depths of Clarissa's character. By providing a completely different character in her novel, she managed to show us parts of Clarissa that we wouldn't have picked up otherwise. This goes back to her belief that characters are the devices that drive the story and not the plot.