In the book, there are five people witnessing the killing of Carlo: Michael, Tom, Rocco, Clemenza and the driver.
In the film there are at least five more witnesses. Why?

The family had a lot of buffas.

Exactly. But the buffa-mechanism wasn't used. Every-one could tell that Clemenza was the killer, Michael, Rocco, Neri, Tom were present.

Michael could have wanted to send a visible message to deter any other potential traitors.

There are a ton of differences between the book and film. Cinemeatic expression sometimes requires more drama or constrains just how a scene can be staged. I think that's what happened in this case.

Technically, there were only two witnesses to Carlo's actual death: the driver of the car in which he was garotted, and Clemenza. The others, who were in Carlo's home earlier when Michael confronted him, heard Michael say that he wasn't going to kill his sister's husband--his punishment was that he'd be out of the family business.

I love the part when Mike says: You have to answer for Santino, Carlo.

Olivant once again raised the point about cinematic expression overriding logic--in this case, having a witness other than Clemenza to Carlo's execution. But again, what multiple witnesses heard from Michael was that he was banishing Carlo from the family.

A couple of far more serious violations of logic occurred in the films:

--When Michael, Calo and Fabrizio are at Sr. Vitelli's cafe, he tells an indignant Vitelli and his sons his name and that he's hiding from the police of his country--not real smart things to reveal to three guys who think he's intending to commit the ultimate Sicilian infamia of dishonoring their daughter and sister. He also tells Vitelli, "There are people who'll pay a lot of money for that information. But then your daughter would be losing a father instead of gaining a husband." Vito never, ever, would have approved of making an unnecessary threat. It's a great scene--one of the best in the trilogy--but it doesn't make Michael look very smart.

--In the penultimate boathouse scene in II, Michael has a full discussion of Roth's escape plans with Rocco, Neri and Tom present, then says, "I want it [his plane] met." He had three witnesses to a direct order for murder--no buffas. The drama of that scene (yet another great one) was to show just how low he'd go to humiliate Tom and manipulate Rocco. But could easily have been a fatal mistake for Michael.

It's true, Rocco, Neri, Tom heard him saying this. But there were many more people around when Carlo was killed in Michael's presence. Every one of them could witness that Michael did not interfere, that he didn't call the police, that he saw Clemenza doing it. There were just too many people around, there was no buffer-effect.
The book has only Clemenza and the driver as witnesses of the killing. Probably only Clemenza got the order.