Well Temperament is the precursor to Equal Tuning.
The development of Common Practice music required modulating to other keys, especially the dominant. Well tuning allows some modulation, but preserves the distinct tonal colors for each of the 7 Major and 7 Minor keys.
Equal Temperament (20th Century convention) destroys those differences; all keys are equally out of tune (off key), to allow unlimited modulation. Playing Bach on a keyboard with modern tuning does not sound the way Bach (or any other 18th or 19th Century composer) intended.
Most elite choirs sing a cappella, because the human mind gravitates toward exact (just) tuning, while modern instruments, especially keyboards, pull the voices off key, ruining the ringing overtones possible in choral music from the ideal blend of vocal parts.
I didn’t know any of that information about being “on key”.
Thanks for explaining. I am reminded that the human voice is also an instrument of music, as is, without embellishment.