Our characters are more than just the creatures of our story world, they are the lens through which readers see into that world. It is possible to tell a story without using this lens, but the resulting story world can be gray and lifeless. Characters interacting with that world and each other give it color, make it seem more real.
I learned this the hard way.
I had a character who supposed to appear to be an insignificant little man though he was rich, had a couple of influential friends, and once was a secret agent. Despite several rewritings, I could not make him come alive. He was dull and boring rather than the mysterious character I wanted him to be, and even when the information about him unfolded during the course of the novel, it too was uninteresting. No matter what I did, I could not make him or his past three-dimensional.
In desperation, I created a love interest for him. (It seems like an obvious solution, but originally I wanted him to be a loner.) When I began to see him through her eyes and her amazement, all of a sudden he burst into full color.
Using one character’s viewpoint to show another character also allows us to be enigmatic when it comes to characterization. If we as the author/narrator were to describe a character as being kind, he must be so; if another character describes him as being kind, he might be kind, but he also might be kind only to her and mean to everyone else, or he might be abusive to her and she interprets it as being kind because she is not used to having anyone pay attention to her. While learning about him through her eyes, we also learn about her.
In this same way, when we see the story world as the character sees it rather than how we as the creator of the world envisioned it, the scenery comes alive. The day might be bleak, but if the character sees this bleakness and thinks what a wonderful day it is, we learn about the weather, and we learn about her.
And we make the story world come alive for readers. We make readers a part of the story because they identify with the characters. They see the world through the characters’ eyes.