One of the major perks with Game Pass Ultimate is that you also get Xbox Cloud Gaming as a part of the package. With this, you can play games streamed as long as you have fast internet, a smart phone or a browser.
And it turns out this feature has been popular. During Microsoft's financial report for the quarter January - March late yesterday, the Microsoft CEO Satya Nadella said that over 10 million people have tried Xbox Cloud Gaming. Back in January, it was revealed that Game Pass has 25 million subscribers, but how many of these that pay for the most expensive level, Ultimate is unknown.
To sum it up, it seems like a very big percentage of the Game Pass Ultimate users are trying Xbox Cloud Gaming, and it's easy to see why Microsoft puts so much effort into this.