There isn't really a secret menu... and that makes things hard for the partners
The "menu" label implies that all the items are universally known by name to the workers, so making one is exactly the same as making any of the drinks off the normal menu. That is, they're easy.
But, as we've said before: secret menus are urban legends. And Starbucks' is no exception. Websites, like Starbucks Secret Menu and Hack The Menu, may provide you with an amazing new recipe but, as Brandon, a barista in Michigan, points out, the article "gives the drink a special name and fails to mention that there is no actual 'secret menu.'" And if someone shows up and orders a Cotton Candy Frappuccino without knowing what goes in there, there's no guarantee a barista can make it.
But the off-menu Frappuccinos are actually a creative outlet
With its infinite add-ins that allow crazy orders like a "Venti seven-pump vanilla, soy, 12-scoop matcha, 180-degree no foam green tea latte," Starbucks' normal menu is intimidating to the basic customer. But the baristas know it quite well, so the "secret menu" allows them to go off-book and take advantage of the customization options. "I have a love/hate relationship with custom Frappuccinos," explains Ashley, a partner in Virginia. "I love how it opens things up, creatively, for customers and partners alike. The only thing I hate is how sometimes it's something we're not allowed to do, like blend a brownie."