instance, suppose you're recording a vocal ensemble
of six independent voices and an instrument ensemble of six instruments.
Ideally you need at least twelve tracks but have only an eight track recorder
for the production.
solution is to record the six voices using tracks
one through six and then mix them together in a stereo pair to the two
remaining tracks seven and eight. Tracks one through six can now be erased
and made available for the instrumental sextet. (The downside of
course, is a loss of flexibility once the original tracks 1-6 are gone.)