[The Cobbler's Apprentice]

"The more I am able to affirm others, to say 'yes' to them in myself, by discovering them in myself and myself in them, the more real I am. I am fully real if my own heart says yes to everyone." (Thomas Merton, from "Conjectures of a Guilty Bystander")
(Regarding the various Faiths, Merton emphasises that the above will to say "yes" to others does not mean "syncretism, indifferentism, the vapid and careless friendliness that accepts everything by thinking of nothing. There is much that one cannot 'affirm' and 'accept,' but first one must say 'yes' where one really can.")
Therefore, as Merton sees it, the purpose of dialogue is simply to become "more real" and who could argue with seeking such? Has "Reality" simply revealed itself fully in just one Faith, one book, one orthodox interpretation?
Comparative religion is full of pitfalls to the unwary. Before any correlations can possibly be made there is a need to have a reasonable understanding and knowledge of any Faith Tradition. A simple example of false comparisons would be one found in certain discussions between Islam and Christianity. Broadly speaking, for Christianity the fallible Bible reveals the Word, Christ, made flesh. For Islam, the fallible man, Mohammed, reveals the Word, an infallible book. Yet often, Bible and Koran are set side by side, and Mohammed set beside Jesus Christ; awkward comparisons that simply create confusion.
Anyway, maybe others see some purpose?