In light air or in rough seas, the sails should be full. When beating in a stiff breeze, however, and unless there are waves, the sails should be flat. An adjustable backstay is used to bend the mast and thereby flatten the mainsail as the center of the mainsail leech gets looser, opens up, and hooks less to windward. This works only on a fractional rig, of course, where the forestay attaches below the masthead.
When the mast is straight, the mainsail leech is tight and it deflects air on the windward side of the main, bending the flow back to windward. This pushes the stern to leeward, causing weather helm. Applying some backstay tension matches the curve of the mast to the curve of the mainsail, thereby reducing the draft. It opens up the leech and directs the airflow straight astern or even a bit to leeward, reducing weather helm.
If the headsail has roller furling, tensioning the backstay will make the headsail easier to furl and will keep it neater and more compact when furled.
When applying backstay to flatten the mainsail, it is often useful also to flatten the jib by tensioning its luff or moving the jib sheet fairleads aft.