There are many benefits to preaching straight through a book of the Bible on Sundays. Preaching straight through makes you deal with a variety of topics, some you may tend to avoid.
So far, in our Romans series, we have looked at Romans 1:1-2:11 over 6 different sermons, breaking the passage down into smaller parts. Along with trying to hear are the main message of the apostle Paul in each passage, we have addressed these topics which each text has brought to light: conversion, the transforming work of the gospel, apostleship and the role and ministry of the apostles, epistemology (how we can know things, specifically things about God), the resurrection of the dead, fellowship, the gospel in the life of the Christian and the church, evangelism, justification by faith, salvation, the anger and wrath of God, sexuality (including a consideration of homosexuality), religious hypocrisy, presumption.
Now I think these are all important topics to consider, and preaching through a book naturally brings all these to light. Thus, there is a great we value and variety of topics that come about when we preach through a book of the Bible.