  The conventional view of stellar-mass binary black hole (sBBH) mergers is
that there should not be enough matter present to produce a detectable
electromagnetic transient. However, there ARE a number of mechanisms for
producing such a counterpart through accretion of matter from various reserves,
charged black holes, or interactions with magnetic or exotic fields. After
reviewing these mechanisms, we describe what we can learn from multi-messenger
observations of sBBH mergers in the areas of stellar evolution and compact
binary formation, cosmological measurements, searches for charged black holes,
and tests of general relativity and fundamental physics. We end with some
remarks about the need to support both observing capabilities and modeling in
the next decade and beyond.
