Paul has three main prayer requests/commands for the Colossians...
...and one primary concern for their conduct.
Both sets of instructions focus on evangelism, more specifically Paul's ministry and the church of Colosse's ministry to "speak the mystery of Christ."
I think Paul may have closed with these instructions in order to remind the Colossians of their primary responsibility: the Gospel. The whole book has been leading up to this reminder. When Paul explains his prayers for them, his ministry, when he warns them, teaches them, and instructs them in Christian living, when he explains how they should act to one another -- it all boils down to living in a manner that promotes Christ and His Word.
While this would be a great place to start closing out our study, Paul continues Colossians 4 with a long farewell speech. Take a look at this and glean what you can.