The traveller is much more than
Seems ta me that the object is to be able to swing the boom across to the opposite tack without going thru the gyrations of letting out-hauling in the sheet,
It adjusts the angle of attack of the sail overall, and allows different mainsheet tensions/leach tension for the appropriate boom position for the conditions. For example in light air the boom can be held near midships by pulling the traveller to windward, allowing a lighter sheet tension for better shape. Conversely in a breeze you lower the traveller to ease the angle of attack, and can pile on the sheet tension again get the right trim for the conditions.
There are others here who can give you more technically accurate data... but IMO after a good powerful vang the traveller is the next thing on the 'must have' list.