Xbox Game Pass Ultimate already includes a number of bonuses, but now that extends to content inside the games themselves. Microsoft's subscription now includes "Perks" like downloadable content, in-game goodies "and more." The initial extras revolve around cosmetics and characters, such as a gold ticket Mission Pass in Phantasy Star Online 2, ship ornaments in Sea of Thieves and five new Gods in Smite. Still, that could save you money if you're already the sort to buy in-game trinkets.
You don't need to do anything extra to enable Perks, and they'll be available through the the Xbox One (through the Xbox Game Pass tab), the Xbox beta app on Windows 10 and the Xbox Game Pass mobile app. While this probably won't spur you to sign up for Game Pass Ultimate by itself, it might tip the balance if you're hoping to avoid spending more than the $15 per month of the core service.