You answered your own question there. obviously this would be a spoiler and would make no sense for the devs to answer (atleast not in less than general terms other than "to complete the quest").
This question is the same as asking "so why did luke skywalker go to the death star, then afterwards, why did he go to Yavin, then why did he go to hoth, then why did he go to dagobah...etc., etc.". In other words, because of the STORY.