Equal temperament is used to allow playing in arbitrary keys without retuning…for example D and E flat are equally in (and out of tune) with equal temperament.

Equal temperament is used to allow playing in arbitrary keys without retuning…for example D and E flat are equally in (and out of tune) with equal temperament. With Pythagorean or Just tuning, instruments are tuned to a specific key (or set of related keys). Which is to say historically, equal temperament is mostly a result of compositional and performance concerns rather than manufacturing limitations.