I've been running a newsletter for a couple of years, and in that time, I've found that cross promoting with other newsletters in my niche was by far the most cost effective way to grow my audience. Unfortunately it's pretty tedious to find other newsletters that you can cross promote with, and because of that, I decided to make this site.
The aim of this site is to provide newsletters of all sizes (although especially smaller newsletters) with a chance to grow out their audience.