Why does Les think believers need teaching on what happens after the rapture?
Many believers lack a clear understanding of what lies ahead after the rapture, knowing only that they will go to heaven. Because Scripture reveals these coming activities in heaven during the tribulation, God wants His people to know them. This teaching is therefore needed to give them a good handle on what is ahead.
Les explains that he is returning to the activities in heaven immediately after the rapture, while the tribulation is raging here below. He says many believers simply do not have a good handle on what is ahead of them, except going to heaven and that being it. But the Bible has told us what we can look for, and if it is in Scripture, God wants us to know it. So he turns to that aspect beginning in Romans chapter 7.