The importance of the Sioux attack in Chapter 29 of this novel is that it prevents yet another complication that Fogg faces in the completion of his quest. Although Fogg, Fix and Adoua are fine, it is the disappearance of Passepartout that is of massive concern to Fogg, as it places him in a massive dilemma. He has to decide between leaving his loyal servant behind to his fate so that he can have a chance of completing his journey and winning his bet, or delaying his departure by trying to rescue Passepartout in the hope that he is alive and has been taken hostage. Note how the text highlights the importance of this decision:
Phileas Fogg, by this resolution, inevitably sacrificed himself; he pronounced his own doom. The delay of a single day would make him lose the steamer at New York, and his bet would be certainly lost. But as he thought, "It is my duty," he did not hesitate.
Fogg is shown by this quote to be a good and honourable man who is willing to "sacrifice himself" for others, even though this will mean losing his bet. Therefore, if the Sioux attack did not happen, it is certain that something else would have needed to happen that would have created a similar dilemma. Perhaps Adoua would become sick and need to be cared for or some special medicine. Either way, the impact of this section of the novel is to show the kind of man that Fogg really is.