Letters to the editor have long been a staple of campaigns. Many times writing them or getting them written is part of a strategy.
People who read such things are likely voters. Everyone likes to see their name in print in a flattering way instead of in a news story where the message can't be controlled as easily.
Of course not as many people read papers anymore so this aspect of a campaign is a lesser part of the whole process. Still, a well crafted, concise and thoughtful letter is a good thing.
Sometimes campaigns help write the letters or write them outright. However, the best are usually ones from the heart written with care by people who sincerely want to help.
The deadline is here for the primary. I haven't seen too many letters, but I remember years ago a local race would produce pages of them.
Another mostly lost art.