For this answer we turned to David G. Schwartz, Director of the Center for Gaming Research at the University of Nevada Las Vegas and author of Suburban Xanadu, which charts the evolution of the casino industry on the Las Vegas Strip from 1945 through 1978.
If you're talking about the famous tri-form (Y-shaped) casino hotel tower, the answer is simple geometry. Since most Strip casinos are built on roughly square plots (as opposed to the rectangular wedges found on most crowded city blocks), they have much more freedom as far as their shape goes.
The Y-shape wasn't invented by Martin Stern Jr., but he was the first to apply it to the casino landscape when he planned the International, which opened in 1969. Stern, the Beverly Hills-based architect who was responsible for defining much of the Las Vegas Strip from the early '50s to the '70s and beyond, had been given the job of designing what was, at the time, the world's largest hotel by Kirk Kerkorian, with no other directive than to "make it big." The Y-shape was an excellent way for him to build a great number of hotel rooms in a compact space. Unlike building two towers next to each other, each room would be guaranteed a view of something other than a wall of windows.
Stern also had the problem of providing a central elevator core that was accessible to the many rooms on each floor. The Y-shape allowed him to build shorter hallways than if he'd built a longer "two-sided" hotel, making access to the rooms that much more convenient.
Since Stern's breakthrough, the Y-shape has become the default for Strip resorts, with the Mirage, Treasure Island, Bellagio, Mandalay Bay, and Venetian among those following in the International's footsteps. The original MGM Grand (now Bally's, opened 1973) is an L-shaped variant of this model and today's MGM Grand (opened 1993) is a cruciform version of it.
