Outposts are good for keeping watch over an area that is key to security in early stages of the game. They are also commonly used to keep watch over important resource areas to notify players of enemy expansion. They become less common as the game progresses, due to the availability of better scouts such as the Light Cavalry. Also, they cost stone, which is the rarest resource and better spent on Castles, Walls, and Towers. Therefore, Houses and Lumber Camps (which do not cost any stone) are more commonly used to watch an area.