[dragonwriter]

> My understanding is that that's only Catholics

No, it mostly just some subset of Protestants that reject it (the practice is common, as well as in Roman Catholicism and some Protestant communities, in the Eastern and Oriental [despite etymological similarity, these are not synonyms] Orthodox Churches, and, apparently, the Assyrian Church of the East.)

> and, only for those in purgatory.

It's true that in the Catholic tradition, there is a special connection between prayers for the dead and purgatory (arguably, purgatory as a doctrine is a Catholic explanation for the ancient Christian practice of prayers for the dead.)