What Is The Author's Purpose When They Use Sarcasm? (Other Than To Add Humor To The Writing)

While this sounds suspiciously like a homework question, which I avoid, I am a writer, so I will do my best to explain.
there are many uses of sarcasm in writing.
In humorous books, it will obviously add another layer of humor. In other works though...
Sarcasm can be used to make a character seem cynical.
Sarcasm can be used to add flavor, to keep a book from being a book full of more of the same.
When written in the first person, sarcasm can be used when the main character is under several extreme emotions. For example, when faced with a horde of zombies, while the book might be very scary, sarcasm can be used make the character seem real. If you saw a Chinese zombie eating a dog, you might think that it was typical, right?
Sarcasm can also be used to help irony along.

In short form, writers have been using sarcasm to make books a lot more interesting a real to readers for a while now. I'm sure if Dickens had used a little, maybe we might not have groaned when we were forced to read David Copperfield in High School.
The purpose is to add flavor and to make the story seem more real to the reader. Since sarcasm is easily recognized, it helps readers connect.

The same reason one uses it in real life.

I think the character is trying to be sarcastic and they r speaking to you in  a way like a diary because I do that a lot.