Read this: The theories surrounding equal temperament began to be put in wider practice, especially as it enabled a wider range of chromatic possibilities in hard-to-tune keyboard instruments. Although Bach did not use equal temperament, as a modern piano is generally tuned, changes in the temperaments from the meantone system, common at the time, to various temperaments that made modulation between all keys musically acceptable, made possible Bach's Well-Tempered Clavier.
Now answer this question, if there is an answer (If it cannot be answered, return "unanswerable"): What does equal temperament enabled in hard to tune keyboard instruments?
a wider range of chromatic possibilities