they aren't going to read any of it.

This is how I understand it:

Every single piece of delivered communication must be read. Not necessarily read by the politician you're sending it to. That's what (some of) their staffers are for. Interns are mighty useful too.

There's a whole list of things they can use to filter a piece communication so that it never reaches the politician.

Threats are cataloged separately to be referred to if there's ever an incident, believable ones (assassination, bombings, etc) are followed up on immediately of course.

Abusive language alone whittles it down the huge pile significantly. I don't know if they're allowed to dispose of them or if they have to keep them for $n time. If you want your letter to be ignored, this is the perfect way to achieve it.

Everything else gets cataloged and analyzed for the staff to debrief the politician on what the plebs are thinking.

Useful stuff that helps a political point (eg "Thank you Obama, my kid's life was saved because of the ACA!", or "I lost my house because of the banks, please help!") gets put aside as a candidate for talking points. The staffers then pick out the most well written or most poignant of these, and that's what your politician hears about or uses in speeches.