Paul likely included this so that the Thessalonians would understand that these are evidences that Christ's return is near, but it had not yet happened. There had not been a falling away, and the man of sin had not been revealed. Paul was gently giving them evidence by which they could evaluate that the return of Christ was not immediately around the corner. As they began to analyze it, this might have been quite discouraging to them. Nevertheless, in AD 51, the return of Christ was very much on the minds of church members.
In addition to this, II Thessalonians 3 deals with the issue that some in Thessalonica had quit their jobs because of misinterpreting Paul's sermons, and they became busybodies while waiting things out. This is not good.
On the one hand, we are to live our lives always anticipating Christ's imminent return, partly because we do not know when we will die and our judgment ends. On the other hand, we are also to live and work as though this world will never end. Since nobody knows when Christ will return, we are to do our jobs with all of our might, as Solomon says (Ecclesiastes 9:10), and because we serve the Lord Christ, as Paul writes (Colossians 3:24). We are to do every job as well as we can, not carelessly cutting corners, assuming that it will all blow away in just a year or so. Such an approach is not a godly attitude.
The people described in Thessalonians 3 were just waiting things out because they had it in their minds that Christ would return almost immediately. That is sheer presumptuousness, carelessness, that God certainly does not appreciate in His children, because that is how He does things. We are to imitate Him. We are to work as He does, and His handiwork is all around us. He does pretty good work! The things that He builds last. They are high-quality work.
In practical fact, there is a tension between the two extremes that must be balanced. Jesus says, "My Father has been working until now, and I have been working" (John 5:17). We are to imitate them materially and spiritually. Those simply waiting things out then were castigated, and time is not waiting for us either.