There are many reasons that you would want to write letters to the editor. One such reason is this:

You can use letters to state your opinion about any kind of an issue or a story that you seem to think the publication is ignoring, for whatever reason. Sometimes all it takes is a single letter from a reader to start the ball rolling and to get more letters coming in on the same topic. If the editor is sitting there seeing that they're getting three and four and five, maybe a dozen letters to the editor on a topic that they've never had a story on, that shows that there's interest in that topic. They might assign the reporter to do a story on it, in addition to printing the letters.

You can find more reasons like this and more great information in How to Use Newspaper and Magazine Editorial Pages.