So 1st person is the I/We viewpoint, while 3rd person is the He/She/It/They perspective.
By using the I/We viewpoint, the author can tell a story from a more personal perspective. Saying, for example, "I walked into a deep, dark tunnel, with no sign of light at its end," allows the reader to identify with it, giving the sentence a much more personal feel. By using this, the reader is better able to identify with the author and his perspective, feeling that they are viewing and experiencing the story writing themselves, rather than just being observers. If the author wrote this in 3rd person, saying "He walked into a deep, dark tunnel, with no sign of light at its end," the reader would feel like she is more or less observing someone else doing these actions. 1st person is a stylistic choice that an author can choose to employ to allow the reader to feel more immersed in the story, as if the events are happening to them instead of to a 3rd person that they are reading about.
Hope this helps!