In this snippet, Stella tries to escape a trap but fails, and Walliams describes how that makes her feel.
Understanding how she feels brings us closer to her, and makes us more involved in the story, because we start to feel the same way.
But here's an important point: other characters in the same situation might feel a different way! A different character in this situation might not feel dejected, they might feel angry, or sick, or calm. Different characters can have different emotional reactions to the same situation.