I’m not really going to cover the millennia of this word’s usage; there are whole books on the subject. Instead, I’ll concentrate on the word itself and on pieces my choir has sung that include it. I will include a brief foray into its usage in Shrek, though, so keep going to the end if you want to get that!
It’s stunning to look at the pieces that are either totally built around this one word or include it as a significant part of the lyrics. Why is it so powerful and attractive? Such questions are always in the end unanswerable. I’d postulate, though, that the sound of the word itself and its use as an exclamation of praise can claim at least partial credit. Why are there two spellings, by the way? Very simple: “hallelujah” is a transliteration from the Hebrew Old Testament and “alleluia” is a Latinized version of the same word in Greek.