The point of the traveler is to be able to control the angle of pull on the main sheet. Picture it this way; the sideways force on the boom is pretty much constant at whatever level is needed to keep it at the proper angle. If the traveler is to the windward, the sheet is pulling mostly sideways and there is little downward component to the force. If you move the traveler leeward, the sheet becomes mostly downward and you are now pulling the boom down and flattening the mainsail.
In other words the traveler position controls the shape of the mainsail for any given angle of attack.