In cases where there are a mixture of chemicals and phases, this is a useful concept.

Okay, yeah, the research was abandoned when the war broke out, but it's still a useful concept.

This is a useful concept although as Dunn et al.

Conductance is also a useful concept in the design and study of vacuum systems.

This is a useful concept among people who often work at night and/or according to no fixed schedule.

Do you think evil is a useful concept in this context?

They surely are useful concepts, but we may be foolish in the extreme to take them too seriously.

There is a useful concept in psychology that, I think, makes it much clearer what he's speaking about.

An altogether useful concept, Winston thought, most particularly in time of trouble.

Time seems to be a very useful concept in making the world make sense.