Why is a brief overview of the Book of Revelation included or important to this Bible study?  

As mentioned in Section Three, John describes  in the Book of Revelation, the one Bride of the Lamb, illustrated as the one New Jerusalem, who’s foundation was made up of two constituents (1) the twelve tribes of Israel and (2) the twelve Apostles of the Lamb. Also in the Book of Revelation, John described God’s elect as two constituents: (1) the people of the Commandments and (2) the followers of Jesus. 

Throughout Scripture the two constituents of believers are identified as:

Jew & Gentile

Two Olive Trees

Two Witnesses

Two Sticks

Two Candlesticks

Those who sang the Song of Moses and those who sang the Song of the Lamb

Those who keep the Commandments and those who follow Jesus

Twelve Tribes of Israel & Twelve Apostles of the Lamb

Now that we have a working knowledge and understanding of the identity of the Singular Bride of Christ, who is both Jew and Gentile, my effort will be to show-case that the Bride of Christ's presence in the earth during the tribulation. 

In the Spirit of full-disclosure, God has given us the The Book of Revelation as a preparational guide book of what to expect and when to expect these descriptions and most importantly, to receive His instructions as the pre-Script of our role to play!