The structures are the buildings that meet guest's needs, such as hunger, thirst, restroom need, and energy, some of the structures satisfy only one of this needs, like the food and drink stands and restrooms, while a restaurant satisfies the four together. In other cases, the building is for a guest's happiness bonus
Also most of them are a source for the economy for the zoo. When the player sets a new building, it has a default price, but this can be set to a lower or highest price (except the compost heap). In some cases the guests will complain about high prices, and this will deter guests to enter, the exceptions are the restaurants and souvenir store, in this cases the player can set maximum price and the guests don't complain.