And while I don't want to pick on svsephyr44, the majority of the items that he stated in this thread are in error or misleading. It would take more time to explain than I have this evening. But basically the traveler, working in concert with the sheet, controls the twist of the sail. Being able to control twist is critical to sailing in light air, where the impact of gradient wind effect is greatest, and to safely sailing in heavy air, controlling heel, leeway, and drive. Travelers are especially helpful on split rigs since they can help control down wash (bad air) on the sail operating to leeward of that sail.
I am sorry that you didn't have the time to correct those items that you thought I had stated in error or were misleading. That does not help people reading this thread.
"Twist" simply refers to the changes in the sail's chordlines. Or in simple English, look at your battens. In absolutely dead air, no waves, boat not moving they will all be stacked up parallel to each other. As the wind starts to blow they will no longer be parallel. This is in part because of wind gradient, the fact that the amount of force with which the wind is affecting the sail, varies from the waterline to the top of the mast. The sail is no longer perfectly flat above the boom, the middle of the sail between the boom (the foot) and the head will be pushed out to leeward. So the aft end of the middle battens are pointed further out to the lee than the top and bottom battens. (Of course the front end of the battens are still in a line as they are firmly (we hope) attached to the mast.)
Twist is a function of the relative tension on the clew in two dimensions, down and parallel to the deck. Pull a sail down with no back force and it will balloon out. Pull a sail back with no down force and it will get very flat. (Note that we are not keeping a fixed attachment point on the deck here - I am assuming you are pulling the sail down from directly under the clew or back from an infinite distance behind the boat.
An easy way to see this is by moving the jib cars. If you move the jib cars forward you are increasing down force and releasing back force that is, tightening the leech on the jib. If you move the jib cars aft you are increasing back force while decreasing down force, in essence slacking the leech of the jib. So try it and look at the difference in sail shape. Most people have telltales on their jib. The purpose of the telltales is to get twist right. When all the telltales are streaming back and popping (breaking?) up the entire sail is at its most efficient. Using the telltales and adjusting the jib cars gets the twist right, in other words gets the leech tension on the jib at its most effective (efficient?) point.
On a mainsail on a masthead rig the golden rule is to use leech tension to get the top batten parallel with the boom. One way to get leech tension is using the relative position of the mainsheet and the traveler. A second way is to use vang tension. On most boats in real time the actual trimming (we are not racing here) is a sloppy function of both.
With respect to Jeff - who I infer actually has a lot of knowledge and knows what he is talking about - "perfect" sail trim is impossible to achieve (maybe like in Camelot you can do it "for one brief shining moment.") In the real world you only have (for example) a main and a jib hoisted at any particular moment. If both sails are perfectly trimmed for maximum generated force (perfect trim) then the boat is getting maximum "drive." ("We be racing grandpa!") But is maximum drive desirable for the actual wind and wave conditions? Or do you want something less? In heavy waves you might prefer to go a little slower rather than beating the heck out of the boat and yourself. So you depower the sails (by letting the sails spill some of the air rather than proving maximum drive (i.e. changing the twist) and slow the boat down.
In addition, forward speed is constrained by theoretical hull speed and the impact of wave action. When you hit a wave the apparent wind changes. When you surf down a wave the apparent wind changes. If you bow gets knocked sideways by a wave your apparent wind changes. In anything but flat seas and absolutely constant wind your perfect trim becomes wrong within the first second.
When you change the twist you are "powering up" or "powering down" the sail. Why power it up or power it down? Because the wind speed varies and you only have one sail. Sometimes the wind gives you too much force, sometimes too little force. Since you don't have an infinite number of sails that can be changed out each nanosecond one adjusts the efficiency of the sails to take into account the current conditions and not over stress the rig. Is a mainsail whose top batten is pointing to leeward improperly trimmed? I don't know. Perhaps a purist would say yes, after all the "objective" is to get that batten parallel with the boom. But if getting the batten parallel to the boom means that I am overpowered, crashing the boat and straining the rig I will accept imperfect trim.
My objective as a long distance ocean cruiser is to get my sail plan at about 85% to 90% of optimal. When solo sailing 24/7 for several weeks at a time my desire to "man the winches" is non-existent. I have to do it often enough when the weather or wind changes thank you very much. I would rather reef than power down my sail plan. I will accept less "drive", less "leeway", and less "heel" to enjoy a cup of coffee while petting my cat
and watching the sunrise. When sailing with a Monitor wind vane (a device that steers the boat on a constant angle to the apparent wind without using any power) my most critical sail tuning dimension is getting balance as the Monitor hates fighting an unbalanced plan that is causing lots of lee or weather helm. My objectives were very different than when I did long distance racing with a full crew.
My objective in my original post (the one that Jeff takes issue with) was to simply enlighten readers that a traveler is not just a hunk of metal on the deck that one can get around to understanding when they get around to it but rather an important and effective part of the overall sail control system. I do not hold myself out as an expert or an experienced sailor. I was not trying to write a book on sail trim - I do not feel qualified to do that. I just wrote some off the cuff thoughts about how I view trimming my boat. I am just a guy that has done his 4 hours of learning to sail and am now spending my lifetime trying to learn to do it well.
Fair winds and following seas