An older grand organ wouldn't need to be re-tuned if it only ever played on it's own. Imagine re-tuning each and every pipe.

Instruments such as fiddles, flutes and guitars can change their tuning easily. In tunes sessions fixed pitch instruments such as concertinas and melodeons set the pitch for those that can re-tune. If instruments don't play together they don't have to have the same pitch.

Re-tuning concertinas can be a difficult task. Reeds have a different pitch in the instrument to when they're out of it but they need to be out to be re-tuned.