The title was created because the two brothers' spheres of influence is over the spirit. Irmo is the master over visions and dreams, Námo has power over the fëar of Elves and Men who gather in his halls after death, where the fëar of the Elves abode while those of Men leave the Circles of the World. Námo delivers judgement on Manwë's bidding in serious matters (such as the Doom of the Noldor) but also judges over the fate of the elvish fëar.
Both are more commonly known by the names of their dwellings, Lórien and Mandos.
In contrast to Thanatos though, Námo's role has been expanded. In greek mythology Thanatos is Death himself, but he neither judges the dead nor does he rule over them. Thus Námo also resembles Hades the greek god of the Underworld.