Development and design working together makes better products for our users. Design and usability decisions have a big impact on the developers who implement them, and, ultimately, on the experience of users. For these decisions to be successful and provide users with the best experience, communication between designers and developers is vital.
When developers are expected to work in a corner until needed, that isolation from the design process prevents them from crafting the end product just as much as the designers themselves. The person who ultimately pays is the user.