Final Fantasy XIV is meant to destroy items rather than make new ones, although you can end up making new ones after you break them down. It's a chance to take your crafting skills and apply them to break down items that you have no use for, making something better from the resultant materials.
Desynthesis will allow the breakdown of items by crafting classes above level 30, who will recovering some of the items used in its construction. Skill level affects the odds of a successful desynth; there is a maximum skill level shared across all crafting classes, which will prevent players from being equally skilled in all crafts. More to the point, breaking down an item can unlock materials that can't be acquired through other means, allowing players to make even more powerful items than would otherwise be available.