"Therapeutic"? Yes ... the square root of minus one. Transfer functions, gyroscopes, tacho-generators, Metadynes et al - that all takes me back a bit. Oh yes, "proper" engineering!

I should imagine that power factor correction is still used in large factories (if we have any left). Anywhere that uses lots of big motors running more or less continuously. Cos theta rings a bell.

Big banks of capacitors. You sometimes see them in electrical sub-stations.

If you don't inspect ... don't expect.