GF III establishes a few things:

In the 19 years since Hyman Roth's assassination in 1960, Michael Corleone has:

-Amassed roughly at least a billion dollars in personal and corporate wealth, enough so that he can easily cut a check for 200 million

-Removed himself from having any direct involvement in Mafia life, except as it seems an elder statesman of the Mob and Commission member. Nothing in the film indicates any direct involvement.

-Has become in these 19 years well known as a philanthropist and Wall Street level businessmen, such that only a few in the media even bring up his questionable past.

-Has successfully rehabiliatated not only his personal image, but that of the Corleone name.

-Has sold all the casinos, and presumably has amassed most of his wealth from both ownership and sales of the Casinos, as well as his (mentioned in GF 2) shares in the stocks of IBM and IT&T.

-Has become "clean" enough to be awarded one of the highest honors a layman can be bestowed by the Catholic Church.

As such, why does he seem to obsessively want/need Immobliare? The film's reasoning is that the purchase would "clean his family name"....But given all the things listed above, wouldn't he and his family name already be clear? It's never quite clear WHY he wants Immobliare, or what he intends to do with it once it's gotten, other than pass it down to his children.

But that circles back to point 1 - he already has at least a billion dollars to pass down; why spend 600 million of it investing in a real estate company?

It can't be to fund a Presidential or Governorship for his family; His son wants no part of being a lawyer, Tom's son is a Priest, and his daughter is a female in 1979 with seemingly no practical life experience or public or political experience. So, he's not amassing more wealth to push for a "Senator Corleone" as his father dreamed of. He's not amassing wealth to be able to "buy himself out" of the Mafia; that was what he was doing in Godfather II already. It can't be to cleanse his family name; he's done that.

So why? This is the device that drives the entire plot, and it makes no sense to me.

In GF I, what drives the plot is both Vito's refusal to deal in drugs and its ramifications, as well as Michael's personal character arc and the passing of the torch from father to son, from one age to another.

In GF II, what drives the story is Michael's expanding casino business interests, which make sense on several levels and mirror the Mafia's takeover of Vegas in real life as well as the chess match between him and Hyman Roth as well as the further isolation of Michael from everyone else emotionally.

But what is the point of Immobliare?

Beyond that, what is the point of GF III? He's already a changed man - kindler and gentler - wracked with guilt and desperate to rebuild a connection with his family. He's already a man cleansed of his past sins and is a legitimate businessman, which is what he always wanted to be. So what the fuck does Immobliare have to do with it?