Thanks for the questions. Sorry to have only cited part of your email, but I think I have copied enough for other readers to understand your questions. My answer has several parts so I may only just begin my response here. We'll see what I can do, because they really are excellent questions and go far beyond what I was thinking of when I read your first sentence! They are especially important during Advent because they (like the season) prepare us to celebrate events that are at the heart of our faith, events that stand in the past and present, and events that also stand before us as our future.
The first reason we tell these stories then, is because, unlike many other religions, Christianity is built on historical events. Christianity does not theorize about what God is like. Instead, it reflects on the God whom Jesus revealed to us during and through his life, death, resurrection, and ascension. Other religions might consider God as a creator or hypothesize that God loves us, or that God judges sin and sinners, but Christianity begins with historical events and everything we say about God must be rooted in these events. We know that God has power over life and death because the events associated with the Christ Event reveal this to us. We know that God is a creator God because he is revealed as the author of both new and old creations in the Christ Event. We know that God says no to sin and yes to sinners, and so too, that God's judgment is a sort of harvesting that teases the weeds (sin, false self) and wheat (true self) apart because that is how Jesus dealt with sin and because God vindicated Jesus by raising him from death to new life. Everything we claim to know in Christianity is dependent upon the historicity of the Christ Event and whatever we claim about God, the One Jesus called Abba, we know because Jesus revealed this to us.
The second reason we tell these stories is that at our core we are storytelling people. Telling stories is our way of communicating deep truths, dreams, aspirations, reminders of who we are and what we are made for --- all of this and so much more are communicated through story. (cf., Advent Decisions: In What Story?) Our family stories provide a context for new members to truly belong more deeply; we share the most meaningful stories of our lives with those closest to us to invite and draw them more intimately into our own lives. We listen carefully to the stories others tell us because in this way they gift us with a place to stand in their own lives --- a place we could never have shared otherwise. God has offered us a place to stand in His Own story and that place is given to us when we take our own place in this ever-enlarging narrative the Church recounts for us each year throughout all of her feasts and liturgies. During Advent we prepare ourselves for hearing stories we may have heard many times before, and we prepare because at the same time we also know that we have NOT heard this year's story before and we do not want to miss anything this time around!!
The third reason we celebrate Advent and tell these stories again is because they allow us to understand what God, in Christ, has begun to do and continues to do day in and day out here among us and with our world, namely to recreate this so that heaven and earth interpenetrate one another so completely that God who is Emmanuel (God with us) will be all-in-all. We look forward to the day when Jesus "comes again" (parousia or παρουσια, pronounced pah-roo-SEE-ah with each syllable separate from the others), and we recognize that we are in the midst of a world being remade by God with Jesus as the firstborn of this new creation. So, we celebrate Advent and all the liturgical seasons, not merely to honor the past, but to learn to think and dream in terms of what God is doing now and working toward the world he is continuing to recreate in the Spirit. We do not tell these stories again and again out of nostalgia, but because we find our truest selves in them, and too, the will of God and future of our world and entire cosmos.
And here is where the importance of really good preaching and teaching comes into play. It is up to the homilist or the good teacher to link past events with both present reality and the future that is already coming to be. What we need today are not sentimental stories fit for children alone, but rather, challenging and inspiring stories that give us something to hope in and to work towards. Whether on the macro level, the BIG story of what God is doing in and with God's creation, or the smaller level of our own personal narrative, we celebrate beginnings and newness regularly as an impetus towards growth and fulfillment. Advent serves this purpose on both of these and a number of other levels as well. It provides us with a story we can stand in as human beings in a troubled and troubling world and at the same time it reminds us that in the Christ Event God was and is doing something that affects the entire cosmos.
In today's Gospel story, for instance, we find God taking a young girl overshadowed by the Holy Spirit so she becomes pregnant with Jesus; this young girl then marvels that nothing is impossible with God!! That this story is historical is important for our belief and for our capacity to hope, but even if this story were mythological it would convey something of the nature of God's love that only a story could convey adequately to other human beings. The power of God's love to overshadow and transform reality is something we hear about again and again in the Scriptures. The choice these stories place before us is one of hope versus despair, life versus death, meaning versus meaninglessness, and the power of love over the power of carelessness, bigotry, and hatred. Yes, the Church is encouraging us to believe in a God whose love for us results in miracles and more importantly, results in an ordinary world that is itself extraordinary with an even more extraordinary future that calls for equally extraordinary commitments to life, love, and hope from us. The stories we rehearse are not for children (though if they capture our imaginations as children that is a wonderful thing); instead, they are for adults who can commit to working for a Kingdom where Christ is sovereign and God is all in all, adults whose hope must be rooted in history in the same way it will be fulfilled.