I allows easy limited adjustment of the slow running mixture. It should have been set up so that the knob is in the middle of its range when the mixture is correct
If necessary to reset the mixture pull the knob off and set the mixture screw following Joe Reeves instructions on the Engine FAQ board or via the "Top Secret File" in the stickies on this board, then refit the knob.
But if its running right don't mess with it. Just leave the knob in the middle of its range.
(I use the knob sometimes when starting, but this is just my way.
I find if set to its minimum I don't have to be so quick getting the choke off when starting from cold.
and if set to maximum I don't need the choke if starting when slightly warm )