The economic system is surprisingly deep. You can buy out legitimate businesses, invest in weapon upgrades, and even bribe the police to ignore your wanted level. It creates a power fantasy where the streets literally bend to your will.
This narrative device allows the player to exist "in the margins" of the film. You are the guy holding the door open during the meeting where Sollozzo meets Don Corleone. You are the one driving the getaway car after the assassination of the Turk. You are the one planting the gun behind the toilet for Michael. The Godfather- The Game