You need this for Drop braces CTRL cases. The two pieces won’t fit together without it. If you already have a diffuser from an older Massdrop branded (it says Massdrop on the bottom) CTRL case, that won’t work you still need to buy this.
Why doesn’t Drop just include this with their cases instead of selling it as an add on? If you don’t know better, it’s easy to miss that you need it. It also feels like Nickle-and-diming to have to pay an extra $10 to get a $145 case to work.