Eph 3:1-3

Ephesians 3:1
For this reason I, Paul, the prisoner of Christ Jesus for the sake of you Gentiles—

Paul highlights to the Gentiles (non-Jewish) that he is paying a high price to be their “champion”. Paul believed that everyone had a right to the gospel of Jesus, whether a Jew or non-Jew, and for this conviction, he would often be imprisoned.

Paul says that he suffered that for them.

Ephesians 3:2
Surely you have heard about the administration of God’s grace that was given to me for you,

Paul may have mentioned the suffering he endured on their behalf to confirm his credibility to those Christians who hadn't heard about him, using his scars as evidence.

Beware of leaders with titles but no scars. But at the same time, understand that the majority of people carry scars that you may not know about, so do not be too quick to dismiss or promote.