I agree with this: "just a percentage without context makes no sense. You reduced costs by 16%? Costs of what? What's the denominator? You increased engagement? What does that mean? Subscribers? Clicks? Time spent?"
And this: "Charitably, it's to show you care about business context."
But disagree that you should not do this if writing resumes for "numerate, technical types."
A great technical resume helps get you the interview. In an interview, especially a behavioral interview, you'll be asked deeper question about that 16% cost reduction. Exactly the question you've pointed out. And your resume is more likely to get selected for the reason you pointed out - it shows that you care about business context, or, more broadly, commercial technical achievement.
With technical, numerically literate types as interviewers, they are going to want to know something about how successful you were in being technically accomplished. And the plain, observed, fact is that professionals who demonstrate, with numbers, their accomplishments, get more interviews than professionals who paste their job description, or simply assert they achieved something good without quantifying it.
And a good Paul Graham citation is always welcome and relevant! :)