It's a microcosm of the film, as well as the city. It's a visual metaphor. Sin City, the 'City of Lights' (not Paris!), and "The Entertainment Capital of the World", is ever in the background. It's based on the idea of being an adult playground, where ordinary rules don't apply, all the fun with none of the consequences. It's a bright dazzling spectacle, a show, it's diverting, entertaining, while it lasts, but once it's done it gets dark, the tourists, they and we all go home. Drinkers sober up, gamblers count their losses, tourists get their cabs and catch their flights and go home. It's back to the real world. In a word it's the end of the fantasy. Maybe that is why it seems like such an abrupt tonal shift.

In a little more detail...

Danny Ocean is setup as the sort of trope gentleman thief. He chooses targets of dubious integrity and concocts a scheme that results in no one being really harmed. It's more like an elaborate adult prank of gentlemanly showmanship. His real target isn't the money, of stealing his wifes heart back. So he's really more a romantic gentleman theif. That aspect, as well as the setting is missing in the next two films, and probably why they aren't nearly as good.

Isn't the casino insured too? So then no one is really out the money? Even if it's not, Benedict is rich, and IIRC it's implied it will only really result in embarrassment, and maybe a little personal humiliation with respect to Tess. His hubris is being too serious and grim and can't outmatch Oceans playfulness and levity.

At the end of the film, by the fountain, all the 'good' guys have won, got what they wanted, and that's it. Men have enjoyed imaging what it would be like to be Ocean or Ryan or whoever they most like or identify with, women get to imagine what it is to be wooed and fought over like Tess. The film is offering the magical feeling of 'what it would be like to be a winner', just for a little while, especially at the expense of the house. Who needs endorsements and product placement (though they probably had those too) when the movie itself is the ultimate advertisement of the Vegas fantasy?