Do you need a traveler - no. But lets look at the entire setup for a moment. When adjusting a sail you have three objectives:
1. To adjust the "draft." This is the center of effort of the sail and can be moved fore and aft. If you look at the sail the draft is the point in the sail where it is bowed out to the maximum. The location of the draft will influence how much the sail is trying to turn the boat. With a forward draft the lever arm is shorter and the turning force is less. With an aft draft the lever arm is longer and the turning force is greater.
2. To adjust the efficiency of the sail. In lighter winds one desires the sail to be very efficient, in strong winds less efficient. The flatter a sail the less efficient it becomes - hence cranking up the pressure on the outhaul will make the sail less efficient. Putting tension on the halyard or Cunningham also flattens the sail, releasing the pressure makes the sail "fuller" and more efficient. Loosening the vang and taking up on the topping lift will also make the sail "fuller" as releasing the topping lift and cranking down on the vang will make it "flatter." At some point you can't adjust further, that is when you reef or furl.
3. To adjust the angle of attack of the sail. Assuming you have used all the other controls to get the shape and draft you desire you adjust the angle of attack with the sheet. With a single attachment point it may not be possible to sheet the sail into the center line as much as you desire because at the center line the sheet is going up and down rather than sideways. You are adding lots of stress to the rig for that last few inches. By using a traveler to adjust the deck attachment point to the upwind side you relieve a great deal of the down force on the rig making those last few inches much easier to gain and much less stressing of the rig.
A nice benefit of a traveler whose sail attachment point can be adjusted from the cockpit on the fly is that it is a far better way of dealing with gusts than trying to constantly trim the sheet. With enough mechanical advantage on the traveler lines (usually a block with at least three sheaves) you can adjust the angle of attack without disturbing all the other settings on the sail. Traveler down (as you would do by slacking the mainsheet) in a gust, traveler up after the gust has passed. As strange as this may sound with the proper setup it is possible to bring the boom past the center line and on to the windward side. In a strong gust going upwind this makes it possible to pretty much completely stall the sail, a much better outcome than having the boat roll as you scramble to release the sheet. NOTE: remember that with a headsail making it more efficient will cause the boat to want to turn downwind in a gust. You don't want to do this!
More about this below.
In passing, I have seen many boats with traveler tracks with set endpoints and no control lines. This is, IMHO the worse possible setup. When taking tension on the sheet the traveler car slides to exactly the wrong end of the track, the lee side rather than the windward side. If you have such a setup and no control lines pin the traveler car to the center of the track.
Remember that the purpose of all these halyards, sheets and control lines is to get a balanced sail plan providing enough force to move the boat at the desired speed. Most boats sail better standing up then with a lot of heel. Usually excess heel is a function of having too much sail up. "Balance" means that the pressure on the sails is such that if you let go of the helm the boat will continue on the same course. If it turns one way or the other when you let go then you are either getting too much out of your headsails or too much out of your main (and mizzen if you have one.) "Proper balance" will cause the boat to head up into the wind slowly if you let go of the helm. This is a safety factor. The transition from close hauled to a beam reach results in the sails becoming substantially more efficient. It also results in the lateral force increasing leading to increased heal and potentially a knock down. If the trim is such that the boat heads up in a gust the worse (best?) case is that the boat will go in irons until the gust has passed and then fall off to its original course. This is a much safer (and needless to say less stressful) outcome. There are a lot of controls that change balance. Furling or unfurling the jib. Taking tension or releasing the sheets. Tightening or loosing the Cunningham or halyard. Adjusting the outhaul to move the draft. Unless you are racing perfect sail trim is unnecessary. Balance is much more important than that last 0.1 knot of speed. As long as you have the proper pressure to bring you upwind in a gust you are good to go. Don't drive yourself crazy spending hours to get it perfect. If you do the wind will change and it will not be perfect anymore. When racing a good crew is tweaking all of these controls on a regular basis looking for that highest speed combination. That is part of the mystique of racing and can be a lot of fun. But if you are just social sailing you will miss smelling the sea air and seeing to dolphins playing in your wake as you fret over perfect trim.
Going downwind in strong winds you have a different problem With the boom way out to the side when the wind increases you can't ease the sheet because then the boom will hit the stays. You have to get the boom back into the centerline to get the wind pressure off the sail. There are two ways to do this: crank like mad or come about. Neither is particularly advantageous. Cranking takes a lot of effort and time. Coming about puts you at the mercy of apparent winds. Consider heading downwind in 20 knots of wind and 5 foot following seas. Your boat speed is 5 knots. Apparent wind over the boat is 15 knots and you are surfing as the waves pass under your stern. As you come about the apparent wind increases to 25 knots (a 10 knot increase) and you are now crashing into the 5 foot seas. You adjust your main (lets say putting in a reef) and fall off again. Now you are once again trimmed for the wind and most likely ready for a nap! On a broad reach or run in strong winds it is much easier to simply take down the main and let the jib do the work. The definition of a strong wind is one that with a completely unfurled jib permits you to sail at the desired speed without a main. In a strong gust you can release the jib sheet and let the gust go by. When sailing this way you need to be cognizant of the pitch of your hull. If you are going at or near theoretical hull speed on a broad reach or run and the bow is being pushed down you need to furl in enough of the jib to get the boat back level. Without doing so you are providing unnecessary stress on the rig while accomplishing little in the way of boat speed.
I can teach you to sail in 4 hours. Learning to sail well takes a lifetime.
Fair winds and following seas