It was necessary to model the interiors of every building in order to facilitate all aspects of the gameplay that I want to get into the game.
If the player orders one of the their gangsters to extort a business, for instance, I think it would be lame to just have a window pop up saying "You've just extorted John Doe for $200". Now the gangster (or gangsters) actually walks into the business and interacts with the store owner, the player can see this happening. Also, the player can control a gangster and enter a shop and engage in conversation with the owner, or any civilian on the street and get the same results.
I've recently added banks throughout the cities, the player's gang can rob these banks (or any other business) at any time. The skill level of the gangsters, their weapons, numbers, and driving ability of the getaway car driver (if necessary) will all have an effect on the outcome of the robbery. It's a pretty big risk, as in reality...you could earn $100,000s instantly for your gang...but you could also loose a number of gangsters/weapons/vehicles which could potentially ruin a gang's chance for supremecy.
The player can increase the chance for success/a clean getaway by controlling the gangsters that are actually performing the robbery.
Below is an early composite screenshot of some scenes in the game with the interiors modelled. At this point in time I've only got about 15% of the buildings in the city converted to have interiors, so there's not much variety in the city at the moment, once I get more building types into the city, I'll have some real juicy screenshots ;-)