Definition of "useful"

In the early days troubles were experienced with oscillation from the rod drive and with the transformers, but were overcome later, and these machines performed useful service until superseded by more modern locomotives less costly in maintenance.

1960 December, Cecil J. Allen, “Operating a mountain main line: the Bern-Lötschberg-Simplon: Part One”, in Trains Illustrated, page 743

Turbines have been around for a long time—windmills and water wheels are early examples. The name comes from the Latin turbo, meaning vortex, and thus the defining property of a turbine is that a fluid or gas turns the blades of a rotor, which is attached to a shaft that can perform useful work.

2013 July–August, Lee S. Langston, “The Adaptable Gas Turbine”, in American Scientist