It's important to have equally as good interior and exterior so that the player does not lose immersion when playing. In a lot of game all you need is the exterior but games such as Skyrim and Assassins Creed both allow the player enter buildings so a lot more detail and time is put into them. To reduce the amount of time needed developers will create a list assets to be created so every room will consist of those assets but in differnt places making each one unique.