Many reasons: a rehearsal evening can revive flagging spirits, especially under a good Chorus Master. And there might be more personal reasons for staying, too...

Handel Messiah – Hallelujah Chorus, London Philharmonic Choir and Orchestra, conducted by John Alldis. With kind permission from Irving Martin.
© Sparrow / Irving Martin

Anderson Four American Choruses – I'm a pilgrim, London Philharmonic Choir conducted by Neville Creed. With kind permission from Faber Music.
© London Philharmonic Choir