It reminds the reader that to celebrate Jesus' arrival and to follow Jesus is ultimately the path to martyrdom, figuratively or literally. Stephen died for proclaiming that Jesus was the Son of God and pointing out to the Jewish leaders of the day that they'd missed this one and had executed the Son of God.
The easiest solution is to execute Stephen as well. Make corpses while the sun shines, so to speak.
But it really does show that to celebrate Jesus is to follow Jesus, and Jesus died at the hands of the powerful. Stephen is a sobering element to Christmas. The baby grew up, criticised the established order, and was killed for it. And so were his followers.
1. Is that a real and common expression?