An agency developer's hardest problem usually isn't any single client's code — it's holding five different codebases' worth of undocumented context in their head simultaneously, and losing a chunk of it every time they switch. Spec-driven development is disproportionately valuable here, because the alternative to writing it down isn't "I'll remember" — it's "I'll remember until Thursday, when I switch to a different client for two days."
Product identity per client, written once
Running product-design or product-analysis once per client engagement — even briefly — gives you a product.md you can actually re-read in thirty seconds when you pick a client back up after a week away, instead of reconstructing "wait, who is this actually for again" from scattered Slack threads and old email chains.
Design-First is the agency default
Agency work is overwhelmingly Design-First: a client hands you an existing site, a competitor's app to reference, or a designer's Figma file, and the job is extending or rebuilding against that material. design-analysis and art-analysis extracting what actually exists — rather than working from a verbal brief and your own memory of a client call — matters enormously when you're context-switching across clients and can't rely on holding every detail of every brief in your head.
Approval gates as a client-communication tool, not just an engineering one
A spec's draft/approved status maps naturally onto client sign-off. "Here's the approved plan" is a concrete deliverable you can point to when a client later asks for something that contradicts what they already signed off on — and the discipline against silent post-approval edits protects you as much as it protects them; you're not left wondering later whether "the version I built" matches "the version they approved."
One-folder-one-topic across a multi-client repo structure
If you manage several client projects from shared infrastructure or a monorepo, the one-folder-one-topic discipline is what keeps client A's spec history from bleeding into client B's. It sounds like a small naming convention; in practice it's the difference between being able to answer "what did we actually agree to build for this client in March" in thirty seconds versus not at all.
For agency work specifically, the workflow's real value isn't speed — it's that it does the remembering for you, across exactly the kind of context-switching that erodes memory fastest.