The Left 4 Dead blog writes:
"Valve has always seen the value in gathering real-world gameplay data and using it to improve our products. Now we're hoping you'll find it as useful as we do. With the creation of the L4D stats page, our customers will now be able to see a wide variety of gameplay data across all of Left 4 Dead's various game modes.
Use the stats page to track your performance; see which aspects of your gameplay are improving over time, which are your go-to weapons, and how often you use various characters and features."