What was the pit that Will dug used for?

Will digs a pit to char posts.
Will Page is a young boy who has to go live with his aunt and uncle in Virginia after his family dies. The Civil War is raging, and Will has a lot of resentment toward the Northern army for the death of his family. Even his sister was killed, when she got typhoid. On top of this grief, Will has to learn to adjust to his new environment and find ways to cope with the world in upheaval around him. This is one reason why he's confused when he's asked to dig the pit. It isn't a task he has experience with from his past life.
Uncle Jed tells Will that some of the posts around the pasture are rotting. This means that their animals aren't going to be safe. Uncle Jed tells Will that he's going to help with securing the fence. His first task is to dig a pit. In that pit, they're going to char the bottoms of the posts.
Charring the bottoms of the fence posts will make them last longer because it will extend the amount of time it takes for the fence posts to rot. This means that it will be longer before they have to repair the fence again. Since the repair process is going to take a month—according to Jed—this is beneficial to the family.
Will is confused about where to start the pit. He decides to do it near where they're working but far away from dry grass that might start a fire. Once he starts digging, Jed realizes he's confused. He tells Will that while he (Will) hasn't done much work before, people who live around the region do work hard and take pride in it. By the time supper rolls around, Will feels a lot of pride and satisfaction in his work.