As with anything you do with a horse, when you start lunging, start slow, smooth and effortless. Think of a young horse, with a short attention span, still developing knees, and short sessions. (5 to 7 minutes). That's not to say that an older horse wouldn't benefit from roping, but as a general rule, you start with a younger equine.
First and foremost the horse needs to know what is being asked of it and ultimately you both need to be on the same page and "communicate" with each other. So we start with the lasso work to open the lines of communication and create the correct movement with your horse.
It might sound weird to say that having your horse work in a circle teaches him something other than the fact that he can run in circles, but it actually works. Why? Because it establishes the hierarchy between master and horse by controlling the horse's space. It also helps to condition your horse, whatever its age.
Preference communication starts with your horse reading your body language and vice versa. Over time, and with a lot of patience, the horse learns to wait for your cues rather than running like crazy in tight little circles. Once the commands have been learned on the ground, it is easier for the horse to understand them when you are mounted.
And why the circle? How about a square or some other pattern, like figure eight? The circle naturally encourages your horse to use his legs correctly, pick up his feet and place them correctly. And, once you get to the point where you bite your horse, it teaches it to submit to the bit. Of course, you have already done groundwork with the horse, to teach him to give a voice and lower his head when under pressure.
You will have taught him to bite sideways (left and right) and downwards, with as little pressure as possible. Again, with horses, the best way to get a response is to exert as little pressure as possible (and it certainly depends on the horse) and to do it in small learning steps.