I particularly like your point #5, which I probably should have included as a balance to my 3rd point. The New Creation will be filled with redeemed material–both God’s creation and our cultural output.

#2. I participate in Advent, as all the major season and feast days of the Church Year, because human beings must organize time around something. The Romans organized it around the decrees of Caesar. Americans organize it around the State (Mighty to Save), with its decrees defining holidays and feast days. Christ entered time to redeem time, and so Christians ought to organize time around Him. The feasts and seasons universally observed by the Church are a way of keeping time by the life, death, resurrection, and exaltation of Christ, the Lord of Time. The way we keep time tunes our minds, bodies, and hearts to what is important, to Who holds time’s key. The Church Year creates a rhythm of time that attunes us to Christ, the Lord of history. (This is not to say we ignore the State calendar. As a practical matter, that would be impossible. But liturgical time becomes dominant.)

#3. This is probably just an extension of #2, but Advent season, as part of the Church Year, reminds us that how we mark time is a liturgical event. All time is liturgical time. God gave sun, moon, and stars “to mark seasons and days and years.” This is a liturgical reference. Our marking of time is always liturgical–that is, it is a ritual that informs worship. We will ultimately organize time around that which we believe controls time. The Church Year, including Advent, recognizes that time is liturgical, and liturgical (Christian) time is primary.

#4. Advent is a season of preparation and anticipation. You can’t really be ready for any feast (Christmas especially) if you haven’t prepared and anticipated properly. Also, lack of proper anticipation made some people miss the Messiah the first time, while proper anticipation made people like Simeon, Anna, and Mary fit to see Messiah. We do well to cultivate proper anticipation like them, lest we be found lacking when He comes.

#5. Advent (and Christmas more so) is the season of stuff. It is a time focusing on Christ’s great act of materialism: the Incarnation. Advent and Christmas are the perfect cure for Gnosticism, for they revel in the fact that Christ became flesh. They remind us that God made stuff, and He likes it. God is a materialist, nowhere more so than in the Incarnation. He’s preparing a feast for His people at history’s consummation, and there’s going to be a lot of stuff–wine, bread, cheese, fruit, spices…the smells, sights, sounds, and textures will delight and overwhelm. God loves stuff, and Christ came to redeem the material creation. Advent is about that glorious fact. It is about our celebrating Christ’s enfleshment and redemption of stuff, and our longing for His work to be completed. We are reminded to rejoice in the stuff God gives us, give thanks, and be generous, as James exhorted the rich.

And of course there are many more reasons…