The non-responsive and vague letter I got back from Campbell's office prompted me to call for clarification. The only thing clear is that Campbell doesn't have a coherent position on the issue. First, an intern or low-level staffer opens the mail and sorts it by category. Then another staffer types your address into a database and issues you a stock form letter on the subject.
Chances are, nobody who makes policy decisions in the office ever even looks at the letter.
The Senator or Congressperson certainly won't read it. On some issues, the representative might track the numbers of letters and calls received. Sometimes public opinion holds sway with the representative, so an individual letter can make an infinitesimal difference in the official's behavior. The problem with tracking is that it's only useful for issues which can be nicely categorized into pro and con. My letter on Social Security offered substantive information, so it was of course ignored completely.