The purpose was twofold. At least that is what Paul mentioned when he gave con=mfort to the Thessalonians in the first century.
"6 Seeing it is a righteous thing with God to recompense tribulation to them that trouble you;
7 And to you who are troubled rest with us, when the Lord Jesus shall be revealed from heaven with his mighty angels,
8 In flaming fire taking vengeance on them that know not God, and that obey not the gospel of our Lord Jesus Christ:"
Notice that this promise was to them. They were promised relief from the ones troubling them. Reading Acts you can see that it was the Jews that caused the trouble, just as they did throughout the whole areas where the Apostles went with the gospel of grace.
I disagree, Tom. Certainly the Thessalonian Jews were the instigators of the persecution of the new believers, including in Berea, but 2 Thes. 1 reads with a finality.
9 These shall be punished with everlasting destruction from the presence of the Lord and from the glory of His power, 10 when He comes, in that Day, to be glorified in His saints and to be admired among all those who believe, because our testimony among you was believed.
Through both letters Paul has been teaching about Jesus coming for the blessing of the believers & the punishment of the wicked. 1 Thessalonians 5 spells it out.
2 For you yourselves know perfectly that the day of the Lord so comes as a thief in the night. 3 For when they say, “Peace and safety!” then sudden destruction comes upon them, as labor pains upon a pregnant woman. And they shall not escape. 4 But you, brethren, are not in darkness, so that this Day should overtake you as a thief.
There were many warnings relating to AD 70, so that event didn't
come as a thief but the final return we do expect will be unannounced by signs.
Mark 13:31 Heaven and earth will pass away, but My words will by no means pass away.
32 “But of that day and hour no one knows, not even the angels in heaven, nor the Son, but only the Father. 33 Take heed, watch and pray; for you do not know when the time is.
34 It is like a man going to a far country, who left his house and gave authority to his servants, and to each his work, and commanded the doorkeeper to watch. 35 Watch therefore, for you do not know when the master of the house is coming—in the evening, at midnight, at the crowing of the rooster, or in the morning— 36 lest, coming suddenly, he find you sleeping.
37 And what I say to you, I say to all: Watch!”