Many offices also know that it takes a little more effort to print a letter and address an envelope.
Congressional offices have hordes of staffers who do nothing but keep track of letters from constituents. And you don't need to write different letters, the same one can be sent to each Member. And if you save spam, enclose one or two copies of some of the more offensive or obviously fraudulent ones you've received. Let them get a taste of this stuff. As a result, it's important that you write your own unique letter, putting your own spin on the issues.