I believe most of pianists know how to play at least a part of the Well-Tempered Clavier. However, some of them may not know the reason why Bach composed it. The reason behind it is interesting.
In the old time, there are two systems of musical tuning – meantone temperament and equal temperament. In meantone temperament system, each perfect fifth interval is smaller than what we hear on piano in these days. If I compose a piece in C major, it will sound slightly different in G major. However, the piece will probably sound quite different in F sharp major there are six fifth intervals between C major and F sharp major.
In equal temperament system, there are twelve semi-tones in one octave and each of them is equal. It is exactly what we have in these days. Bach’s Well-Tempered Clavier covers all keys. His intention was to show people that we can play the music in any keys and it will sound agreeable.