A lot of story elements in GTA IV are either misunderstood or people don't see how the gameplay contributes to the story. The biggest misconception is Niko is trying to be a good guy and yet him and the player keep murdering and causing chaos.
Truth is though Niko says many times that he feels empty inside, that he's apathetic to what he's doing in Liberty City, and the gameplay reflects that. Running over a bunch of people on the sidewalk won't make Niko burst out in tears, he says a quick quip and moves on. You can go into a burgershot and gun down everyone inside and Niko won't care. His persona and spirit are wounded and tainted, if they wern't his perspective wouldn't matter, nor would him or the player be able to role play as a deranged sociopath.
The activities and what you could do and not do with money was realistic and fits perfectly with the thematic theme. What can an illegal immigrant whose ex military and involved in organized crime do and buy in America? What can a criminal do and buy in America? The answer is simple, empty and mindless activities or products to make you forget how terrible your life is.
Here are the most important things in Liberty City for a criminal:
Guns and better guns
Fast cars
Fast food
Being a criminal in Liberty City is very simple, but the cost of the empty criminal American life will always be negative and scarring. Niko got enough material wealth to buy anything, due to his motives and lifestyle though all that money will be used for empty or destructive things.
For whatever reason though some people don't try to interpret videogames, when you try to think deeper though the experience becomes much richer.