If you read these words without studying the whole letter to which they're attached you could be forgiven for thinking they were just a loving sign-off - a polite showing of affection between a leader and the flock of God.
But that's not what's happening in our text. These aren't just an emotional gush. They're related to the letter preceding them. So this is different from the way we stick a "Yours Sincerely" at the end of a letter where the sign-off becomes a standard close to virtually any content whatsoever.
Here's why this matters. The "God of peace" introduced in the first verse in our text (20) turns out to be the God of a particular kind of peace. He's not merely a peaceful God in the sense of being a pacifist God who never gets angry or acts in serious judgment. As it turns out this is a God of a peace attached to "the blood of the eternal covenant" (20). Our writer wants us to notice this. How often do we naturally link peace with blood-shed? Our writer is identifying the nature of our atoning God's work. Its a peace through the substitutionary judgment of our sin.