bypass port in head

What exactly is the purpose of the bypass port in the a-series head?  What
is the rational for plugging it up?  What does it do when open?  I found
mine to be plugged by rust flakes that had accumulated on top of a small
compression fitting the PO had inserted into the bypass hose to restrict the
flow but not eliminate it.  It was completely plugged and apparently had
been for some time.  What would the effect have been?

Thanks,
Glen Byrns