XD gets you through the stages of design and approval, faster than if you did many prototypes in the final development tool. Once the app or web site is approved you could start from nothing in the actual development tool, and still have saved time.
You can export your XD prototype as a set of images to give to the developer, and as you do that it can generate 1x, 2x, 3x, and HiDPI versions of each element.
If you used UI Kits in the prototype the developer could choose to replace those parts with real UI elements, so that the app would still look correct if Google or Apple change the look of Android or iOS.
In app development terms you're using XD to make and interaction design document, which between the graphics and interactive links, and the comments feature, should let the developer know exactly what they are due to create.
There is no work going on at the moment to have XD export an Xcode or similar project.
Great, so we designers still depend of developers.
Why doesn't Adobe make the next step as it did it with Adobe Muse? To give the tool to the hand of the designer for he creates a website from the beginning to the end without code. This policy would be useful in Adobe XD to allow designers develop real APPS.