[Lindolirian]

If you go along with the Christian view of descent, it would seem that the characters in myths are taking on something terrible so that others don't have to. The sacrifice of Jesus for the sin of the world, Luthien giving up immortality for Beren in Mandos, Beowulf killing the dragon at the cost of his own life. All of them are examples of a single person going the extra mile or "taking one for the team" so to speak.
But I think Tolkien also wanted to convey the idea that things have to get worse before they get better. (Now this isn't exactly hell, but bear with me). In the countless voyages to reach Valinor, many mariners perished and any shipwreck survivor (Voronwe) would tell you that it's hell enough. The point is that there has to be a challenge that makes success just that much sweeter, or actually worth trying. When Earendil endured the Shadowy Seas and made it to enlist the help of the Valar in the War of the Jewels, he showed that the impossible could be done and gave hope to the people of Middle-Earth. The various "hells" of Tolkien's world are most likely there (outside of their basic part in the plot) to teach a lesson to the people who go through them or the ones who benefit from their efforts.