Internet Explorer Developer Channel is a special "work in progress" version of the browser which includes upcoming developer-oriented features and extended standards support.
The program brings many improvements to the F12 developer tools. New event breakpoints and tracepoints are triggered when your specified event fires. The UI Responsiveness tool can now export memory snapshots, so you can restore them later and continue your analysis (or share them with others). You get new icon bar notifications, more navigation options and keyboard shortcuts, improved autocomplete, new event filtering, dominator folding and more.
New WebDriver support enables developers to run automated tests on their web pages and analyse the results.
If you're using WebGL then you may also notice some performance gains here, as Internet Explorer Developer Channel now provides instancing support.
Perhaps the best news is that Internet Explorer Developer Channel is packaged as an App-V application, which means it runs in a virtualised environment and won't affect your existing Windows or IE setup at all. You can run the package alongside IE11 without any conflicts, and uninstall it cleanly at any time.