If you know exactly what you need, then specs are great. Proven solutions for known problems are awesome. Agile is pointless in that circumstance.

But I can count on one hand the number of times stakeholders, or clients, actually know what they want ahead of time and accept what was built to spec with no amends.

When there is any uncertainty, changing a spec under waterfall is significantly worse. Contract negotiation in fixed price is a fucking nightmare of the client insisting the sky is red when the signed off spec states it’s to be green.