"[...] but the alliteration and repetition made it extremely annoying to read."
"Presumably they were essential characteristics in the original Finnish oral history. Human memory seems to be aided by such devices."
Yes, mnemonical qualities were important. For many centuries there was no way to write texts down, they were passed on via oral tradition. But there's something else too - old chants have a hint of magic about them. Or, for a modern reader, an aspiration of magic. See 'shamanism'.
This nice overview has a bit about Kalevala too: