For the plot to make any sense, and the characters' motivations to be understood, takes a certain understanding of Final Fantasy X's odd setting. While Spira is characterized as a vaguely medieval, swords-and-sorcery, Tolkien-esque world, in this case it is not because technology has not sufficiently advanced to progress beyond that point. Indeed, in Tidus' time, the world is seen as brimming with advanced machinery. We eventually learn that Sin is attracted to machinery, and any attempt at reviving the sciences of old draws its wrath. Spira's major church has thus outlawed mechanical objects as heretical, and established as doctrine that it was the ancient civilizations' reliance on technology that created Sin in the first place.
Another standout feature of the Spira is the importance placed on proper ceremonial burials: if the dead are not disposed of in the right way, they will remain on Spira as tormented spirits. Only trained summoners, the same elite who are capable of disposing of Sin, can perform the rites, and Yuna's time on the journey is indeed divided more or less evenly between preparing for the final battle and performing the burial ceremony in the towns and villages she passes through en route.