The answer to your questions will be summed up into one paragraph.
This is:
<span>The great poets commonly take up the subject of death in their works, but it is rare to see a great poet treat death in such a sustained and deeply personal way as Tennyson does. Many of his greatest works were written in the aftermath of the death of his closest friend, Arthur Henry Hallam. </span>
Previewing material is one of the three basic speed reading techniques and allows picking the main idea and important information before digging into details. Previewing strategies are suitable for any material, but are rather used for non-fiction content than fiction books.
You would highlight burns against them and the fire is made ready, the furnace is now hot, ready to receive them, the flames do now rage and glow.
You would highlight those things because God's wrath is being personified as, or compared to, a fire, as revealed in the line the wrath of God burns against them. The fire is made ready... seems like an extended metaphor.
The purpose for including the affair was to have character development in the story. Abigail was willing to do anything, including the attempt to try and kill Goody Proctor in order to obtain John. This also made the foundation of Abigail, Goody Proctor, and John and how they were all significant in story development. If the writer had left of the detail of the affair, we wouldn't know why Abigail was trying to get rid of Elizabeth unless we made blind assumptions.