If you visit the Chambers of the Fayth in FFX after the last talk with Bahamut's Fayth (just before Sin gets available), you receive items, namely rare spheres like Agility Sphere, Accuracy Sphere, Evasion Sphere, among others that otherwise you'd only get in the Monster Arena. On top of that, you get to hear the Fayths out and what they say, which varies from their suffering, Sin's feeling and Ifrit's Fayth even mentions how Jecht came to Spira. There's some exceptions, however. Bevelle is off limits, and Seymour's mother (Anima) has nothing either. Valefor, alongside the sphere you get from listening, has four chests, Ifrit has three, Ixion has two and Shiva has one (I haven't got to Shiva yet this playthrough, but considering the pattern, she'll have one chest). Yojimbo and Magus Sisters don't have any chests (and you can't visit Yojimbo's chamber after the dialogue anymore). Of course, in order to reach Valefor's and Shiva's Chamber you have to defeat Dark Valefor and Dark Shiva respectively.

Did you know of this? I came to discover this some other time I played, but I'm pretty sure it's not a very well known fact of the game, although it's rather helpful, both for story and items.

Extra: Malboros are immune to Haste in FFX. Pretty weird thing to be immune to.