I attended a North Sails seminar last year. The moderator explained as follows, think of the traveler adjustments as "opening or closing the barn door"; basically controlling the angle of attack of the mainsail. If the traveler is set to leeward, you are dumping quite a bit of air; conversely, if the traveler is set to windward, you are capturing quite a bit of air. So, in higher winds you can use the traveler to dump excess wind thereby reducing excess healing & rounding up and in light to moderate winds you can set the traveler to windward to capture more air.
On the other hand, the mainsheet controls twist in the upper 1/2 of the mainsail. Easing the mainsheet induces more twist in the top of the sail allowing air to be spilled from the top of the main and tightening the mainsheet captures more air in the upper part of the main.
Supposedly there is approximately 60% more wind in the top part of the mainsail as compared to the lower part of the mainsail because of reduced friction aloft. Therefore, you are really using the mainsheet to control twist aloft to enable a more symmetrical flow of air across the entire sail, top to bottom for better performance. Of course, in higher winds, you can induce more twist ,by easing the mainsheet, to spill more air from the top of the sail. More twist in higher winds equates to less heel, less weather helm, and reduced tendency to "round up" in gusts/lifts.
I am no sail trim expert; however, these are a few basic things that differentiate the traveler controls vs mainsheet controls. I am sure there are many more nuances that are beyond my grasp.