Waterfall isn't evil.
Sequential plans fit some work very well. Knowing when is a delivery skill, not a betrayal of Agile.
Known requirements
Fits
The work has been done many times and won't change.
EXAMPLE
Rolling out a standard phone system.
Hard sequences
Fits
Physical work, lead times and dependencies that can't be reordered.
EXAMPLE
Fibre installs, office moves.
Fixed commitments
Fits
Fixed-scope contracts and regulatory deadlines.
EXAMPLE
A compliance change due by a set date.
Novel work
Doesn't Fit
Unclear users, new technology, lots to learn.
EXAMPLE
A brand-new customer app.
Source: W. W. Royce: Managing the Development of Large Software Systems ↗
Known path: plan it. Unknown path: iterate it.