Ron Weasley: The Most Underrated Character in the Series
What Ron actually does
Ron's contribution is easy to miss because it's not dramatic. He doesn't have Hermione's brilliance or Harry's destiny. But he's the person who makes the trio work as a unit.
In the first book, he sacrifices himself on the chess board. In the second, he follows Harry into the Chamber of Secrets despite being terrified. In the third, he stands on a broken leg and tells Sirius Black he'll have to kill all three of them. In the seventh, after leaving and coming back, he destroys a Horcrux.
The films cut most of Ron's best moments and gave several of them to Hermione, which is why many people who only watched the films think he's useless. He isn't. The books make clear that Harry couldn't have done any of it without him.
Why he matters to the story
Ron's real function is to be ordinary in a world of extraordinary people, and to choose courage anyway. He grows up in the shadow of five older brothers, he's the best friend of the most famous wizard alive, and his other best friend is the cleverest person in the school. His insecurities are entirely understandable.
What makes him important is that he keeps choosing to stay. The one time he leaves, in the seventh book, the story treats it as a genuine crisis, because the trio doesn't work without him. His return, and his destruction of the Horcrux that feeds on his worst fears, is his coming-of-age moment.
Ron represents the reader. Most of us aren't the chosen one or the genius. We're the person standing next to them, trying to help, and hoping we're enough. That's why he matters.