This is the last verse of this letter, the final exhortation. Many important matters were shared before, but the most important one is at the end.
When we write a long letter, we might also write, or repeat, our most important point in the end. To make sure the addressee takes not of and remembers it.
The Hebrew word translated "amen" literally means "truly" or "so be it." "Amen" is also found in the Greek New Testament and has the same meaning. Nearly half of the Old Testament uses of amen are found in the book of Deuteronomy. In each case, the people are responding to curses pronounced by God on various sins. Each pronouncement is followed by the words "and all the people shall say Amen" (Deuteronomy 27:15-26). This indicates that the people applauded the righteous sentence handed down by their holy God, responding, "So let it be." The amen attested to the conviction of the hearers that the sentences which they heard were true, just, and certain.
One of the central parts of the Bible – although the entire Bible is important and not to be taken apart – but if one had to decide for a central passage, then Romans 3:21-26 is to be considered as one of the most central ones.