Sometimes when debugging or testing, you use console.log() in a loop, or in a callback that is fired rapidly. Usefull, but when you only want to see the live value, your console can be flooded with entries. And even though there is the Live Expression option in chrome, your variable is sometimes not accessable in the window scope. This is where console.observe steps in.
console.observe(var, key);This will create a window object variable window.consoleObserve that will contain updated value of the key you provide. This way you can selectivly create a live expression for your defined key.