Any project which offers a limitless variety of ways in which information can be presented and stuctured can happen both faster and better if there is a way for designers and engineers to collaborate without requiring either group to fundamentally alter their way looking at the world. While it can't hurt to have a designer see source code or a developer open a copy of Photoshop once in a while, environments which let each group concentrate on their strengths grow more rapidly than environments which burden designers with engineering concerns or force their engineers to design an interface whenever they want to alter an engine. The Web exploded in the space of a few years in part because it effected this separation of functions so decisively. An interface can be integrated with the information it is displaying, instead of with the display software itself. This transfer of the interface from 'something that resides in the software and is applied to the data' to 'something that resides in the data and is applied to the software' is the single most important innovation of the early Web.
Good tools are transparent.
Web design is a conversation of sorts between designers, with the Web sites themselves standing in for proposal and response. This is true of all design efforts - cars, magazines, coffee pots, but on the Web, this conversation is a riot, swift and wide ranging. The single factor most responsible for this riot of experimentation is transparency - the ability of any user to render into source code the choices made by any other designer. Once someone has worked out some design challenge, anyone else should be able to adopt, modify it, and make that modified version available, and so on. Instead, with this unprecedented transparency of the HTML itself, we got an enormous increase in the speed of design development.