Backstay tension is something that only really affects fractional rig boats. However, it does help flatten the mainsail and headsail considerably.
I found a backstay adjuster helpful on my former masthead rigged boat. The rig was adjusted so that, when I applied backstay tension, it exerted force downward on the top of the mast, and so that the only way the midsection of the mast could bend was to bow forward, pulling the midsection of the mainsail with it.
You can see how it works by putting one end of a slender dowel on a table, and pushing down on the other end with your hand. The dowel will bow in the middle, out of column. It bends more easily if you start it off with a little pre-bend. I adjusted the mast's rigging so that it could only bend one way, and so that it could only bend so far, and no farther.