For you remember, brothers, our labor and toil: we worked night and day, that we might not be a burden to any of you, while we proclaimed to you the gospel of God. (1 Thessalonians 2:9 - ESV)
Paul has expressed his feelings about the Thessalonian Christians and opened himself up to them, going back to the purpose of opening up about his sharing his life with them... He mentions it here as additional evidence that those who have ministered to them are not like the other traveling philosophers and speakers of Paul's day. And he is using their memories of how he ministered to show this to them. While they were proclaiming the gospel, they did not burden the people with the support of these traveling evangelists. They were bi-vocational... they worked to support themselves... not that it ought to always be this way, but in this fragile time of the first proclamations of the gospel, it turned to be very important.