Dalloway, Virginia Woolf juxtaposes many sub-stories with the main story. She does this to provide emphasis on Clarissa's life. Since neither Clarissa nor any of her family or friends ever meet Septimus or Lucrezia, the plot wouldn't be affected if he were left out, however Woolf chooses to put his story in. Also, the party at Lady Bruton's mansion doesn't involve Clarissa, but it is a part of the novel. The characters in these and other tangents parallel the main characters and provide more background in their life. They give us other people with whom we can compare and contrast the main characters.
The largest comparison in the novel is between Septimus and Clarissa. These are the two most important separate stories, and they are the main characters of those stories. Septimus is the opposite of Clarissa, but at the same time the same as her. All of Clarissa's fears and desires are acted out by Septimus. Clarissa worries so much about what other people think of her, while Septimus doesn't care at all. He does things his own way, whereas Clarissa does what people expect of her. Septimus knows what he wants out of life and does it, but Clarissa can't decide on much of anything. Even though Septimus doesn't act like Clarissa, their surrounding characters suffer similar problems because of them, and act the same way towards them.
Another person who is compared with Clarissa is Richard. They have the same troubles of communication and being understood. Richard wants to say that he loves Clarissa and is so lucky to have her, but he can't. Clarissa also wants to say that she loves life and the things around her, but she can't. They go through life wondering what other people think of them, but can never ask anyone the answer.
The other major character that can be compared to Clarissa is Sally. She is much more extreme than Clarissa's calm nature, but this might be why Clarissa was attracted to her.