The Proper Preface, much like other elements of the Divine Service, changes with the season and specific festivals of the church year. It always begins with these words:
"It is truly good, right, and salutary..."
And always ends with these words:
"...evermore praising You and saying:"
The Proper Preface is a time to locate the congregation's praise in the activity of the season or the day. The most common Proper Preface, used in the season of Pentecost, calls to our attention Jesus' resurrection as He overcomes death and the grave "on this day" meaning Sunday, the Lord's day.
Since God's saving action in Christ is a reality that has been given to us, we therefore join the heavenly host in praising God for all that He has done for us.
The Proper Preface answers the basic question: "Why are we here?" The answer is not complicated. To praise God for all He has done for us. To receive His good gifts.
The Proper Preface is like that moment in A Charlie Brown Christmas, when everything has gone wrong for Charlie Brown and he finally asks the big question, the why question, saying, "Isn't there anyone who knows what Christmas is all about?" And of course Linus stands up and tells the Christmas story from Luke 2, answering the question as to what Christmas is all about.
The Proper Preface answers the big, why question. "What is this all about?" It's about Jesus overcoming death and the grave and by His glorious resurrection opening for us the way of everlasting life.
That's the why. That's the answer to the big question.