When defining relationships, it could be useful to be able to choose
whether each additional match relation was expanding (vs. narrowing) the
match logic.
So instead of
table1.a == table2.A
AND table1.b == table2.B
you might alternatively specify
table1.a == table2.A
OR table1.b == table2.B
Lacking this I must use 2 relationships + 2 portals when I really just want
the result in 1 portal. Without going into detail, this may be useful for
setting up simple m-to-n relationships without an intermediate table.