Black hole seeds probably began to form at high redshift, z ~ 15 - 20, and then participated in the assembly process of galaxies.
Black Hole seeds proceeded through (hundreds to thousands of) hierarchical mergers of smaller protogalaxies. When two galaxies merge into one, their central black holes sink to the centre of the new galaxy, usually find each other, form binaries, inspiral, and coalesce becoming the loudest sources of gravitational waves.
Supermassive black holes grow mostly by accretion, but a substantial number of inspiral and coalescence events are likely to be observed by LISA each year.
LISA will detect coalescence events of massive binary black holes in a wide interval of redshifts and masses extending back to early protogalaxies at z ~ 15. The intense accretion phase that supermassive black holes experience in the QSO epoch erase information on how and when the black holes formed. LISA will be able to directly map and mark the loci where massive black holes form by studying gravitational waves emitted during their coalescence following the merger of their hosts.
Gravitational waves can travel undisturbed and provide clean tracers of the properties of the first black holes: their masses, the time of formation, their number density. This is the information needed to constrain how black holes formed and evolved in the first galaxies.