In this instance it is used for a comparison. It is not always needed as most people will understand that the second part of the sentence is a comparison similar to the first part. You could also word this in this way: Just as day followed the night, happiness followed sorrow.
New words, one handy idiom, and a 2-minute quiz — delivered to your inbox to keep your streak alive.
Anonymous As day followed the night, so happiness followed sorrow.It is not necessary, but it certainliy makes the statement more poetic.
Is 'so' necessary at all?