At first glance, that may not seem significant, but think about who Mark is. Oh, he's that Mark! We believe this is the Mark (also called John) who had left Paul and Barnabas on their first missionary journey (Acts 13:13). This was apparently such a significant failure on Mark's part, that Paul didn't want to take him along when he and Barnabas were planning another trip to visit the churches that had been established. Acts 15:37-38 "Now Barnabas wanted to take with them John called Mark. But Paul thought best not to take with them one who had withdrawn from them in Pamphylia and had not gone with them to the work." Sadly, there was such a disagreement over this between Barnabas and Paul that they split. Barnabas took Mark with him and Paul took Silas with him.
But now at the end of his life, Paul says Mark is useful to him for ministry. What went on in the years between Acts 15 and 2 Timothy 4? We don't know everything, but Mark shows up in Colossians 4:10 and Philemon 1:24 as a companion of Paul when he was first imprisoned in Rome. Somehow Mark must have regained Paul's trust at some point. Maybe it was from his work when he traveled with Barnabas, but ultimately we don't know and apparently we don't need to know.
What we do know is that Paul went from sharply disagreeing with Barnabas over Mark to finding him a useful companion for ministry. You know what that gives us? Hope. Hope for that broken relationship; hope for those who have lost trust; hope for those who have been let down; hope if you have come up short in some responsibility. If you're Mark in this story, you can get back up and by God's grace find usefulness again. If you're Paul, you can have your lost confidence in a person restored. Whatever relationship you view as hopeless, maybe it's not. Of course not every broken relationship will work out this way. But they can. Paul and Mark teach us that. And that reality is full of hope.
"Get Mark and bring him with you, for he is very useful to me for ministry." Sweet words. Hopeful words.