I want someone to explain it to me...
While I'm sure others can provide a more technical explanation than I,
I'll provide an example of where I've found it useful.
I ran a four channel aud tape for a Derek Trucks show. Each set of
mics sound balanced and look balanced in wav form and panning but
with a matrix of the sources everything became panned hard to one
side and wav forms were now unbalanced. Inverting the phase on
one source, before doing the matrix, solved that problem.