You whiteboard an architecture in Lucid or Excalidraw, then translate it by hand into Terraform, service scaffolds, and CI. Every hand-off is a chance to drop a dependency, forget a security group, or quietly pick a different region than the diagram implied. The drawing and the running system start drifting apart the moment the first ticket closes.
There's also no feedback while you design. You don't find out the diagram has a single point of failure or a runaway cost until code review, staging, or the post-incident retro. Gateway collapses that gap. One artifact carries a project from source code all the way to a running, observed system on AWS, so the design and production stay the same thing.