When the engines of a airplane show irregularities that do not reduce performance but indicate a possible failure of the engines, is it useful to climb to a higher level, to increase the glide length that is available in case the engines stop completely?
The problem known in the cockpit is only that the engines do not run like they should. Say they make clearly unusual noises and slight vibration, but loose no performance. There is no indication whether the engines will fail, an in case the do fail, at which point in time that will happen.
It's an emergency, without actual change in the planes performance.
As example, I think of a small single propeller plane, and a situation where longer glide path could be useful. But I think the question applies to any plane that can glide without power.