Question: Why do Jews sing Maoz Tzur on Hanukkah?
Answer: Let me refer you to the new 2 volume work by Noam Zion and Barbara Spectre that deals with almost "every" element of Hanukkah. You'll find a lot of information in this work, including the complete text of Maoz Tzur.
Maoz Tzur is taken from the phrase "Fortress, Rock...." from Isaiah 17:10 according to most. The title is taken from the opening words, as is Jewish custom of the musical accompaniment.
The song is fitting for Hanukkah due to the content of the song. The song reminds us of the history of the Jewish People and our hope for freedom and restoration of self-determination from our foes. It expresses hope for the rebuilding of Jerusalem and the Bet HaMikdash. And the song thanks God for delivering us from Egypt, Babylonia and Haman. Specifically it reminds us of the miracle of Hanukkah. Lastly, it includes a prayer for the restoration of Israel.
The original verses spell out the name "Mordecai." There is a debate as to which Mordecai among the poets and liturgists of that period is intended.
The song dates most likely from the 13th century in Germany. The last verse is often omitted. Over the centuries six new verses have been added, which - as you probably know - are not commonly known and rarely sung. M. Jastrow and G. Gottheil wrote an English version; it isn't really a translation although there is a strong plea for divine support based upon past intervention and a muted call for revenge upon Israel's enemies.
