It looks like a big deal when so many take such a strong offense - rather takes the teaching out of context and magnifies it beyond the scope of balance, lol.
Its important because
1. it is Christological (ultimately, it refers back to God's plan/will, the person of Christ and His identity, the fulfillment of His prophecy and promise, the calling of each to fulfill his/her skopos/purpose, the nature of covenant - as mentioned by Katholikos.
2. we see it in scripture
3. it is a direct historical reality that - like other examples - is a witness to the awesomeness of God
4. we consider Mary the "fruit" of the Old Covenant as evidenced in the Incarnation and the fulfillment of the New Covenant
5. she is the first to have Christ within, and is thus an example of the fulfillment that all seek (and when any of us fail to "hear the word of God and keep it, we fall short of our purpose and carrying forth God's will)
6. because everything about Christ, everything He did and does is eternal, is wondrous beyond understanding, and He "Who cannot be contained" was in her, and "He who could not be touched" was cradled by one, He who feeds all of creation was nursed by her, He who is pre-eternal 'stooped/condescended' to be born a helpless infant. Its humbling, mind boggling: "oh wonder of wonders, how do you nurse the Master?".
7. believing it true, to say otherwise is -simply- distortion and slander
8. many of the attacks on the Church were on the virgin birth (and, of course, on the person of Christ but also of the Theotokos; as usual, the 'quiet' teachings become more focused when defense is mounted against such attacks)
9. the verses requesting the intercession of the Virgin have survived in the EO/OO/RC verbatim (allowing for translation) for at least just under 1800 years when compared to the findings of the early 3rd c. text found in the mid 20th c. (Egypt). Must have some importance to the early and later Churches to survive that long.
10. I'm not sure how to describe this, but (I suspect this is true in the OO/RC as well) I always have the strong sense that - when any of the apostles, Saints, etc are mentioned, we are discussing our family. My family. Whatever the other Churches are like, the EO is familial/community oriented. Christianity is a "way of life", lived out in community.
11. as elder Paisios once said -- when asked about Biblical interpretation -- the Saints interpret the Gospel for us in their lives. All the Saints are "interpretations"; Mary is one.
there's more, likely -- but I've got an early day tomorrow, LOL
Thanks for asking.