Why this works ? Well, most web users have very short attention spans and they just glance at a few elements of your blog page before deciding whether the site is worth subscribing or not.
An impressive subscriber count, displayed at a visible location on your blog, would make the first-time visitor feel that since so many people have already subscribed, this blog must be good enough and he too hits the XML button or bookmarks it. The cycle continues - more subscribers mean more new subscribers.
Feedburner provides a sleek Feedcount web graphic that displays the circulation stats of your blog feed. You can choose any custom color scheme for the Feedburner chicklet from Publicize tab of Feedburner control panel but there's no direct way to change the text displayed inside the Chicklet.
No matter what color you choose, the Feedburner graphic label would always say "1234 readers" - if you love doing things differently - here's a hack to change the text to say "1234 fans" - just append the following piece of code to the graphic URL - "&label=fans"
And here are three variations of the same subscriber count chicklet:
Related: Change Color Scheme of Feedburner Rotating Headlines, Turn Off Comments to Get More Subscribers