Because it is evidence of the what we think of as "supernatural," (though our perceptions in this world are sub-natural, that is, lacking the full spectrum of possibilities). We see only what we are conditioned to see, and this event opens the door to greater vistas.
The orthodox idea is that the Virgin Mary was virgin before, during, and after giving birth, that Jesus birth was totally outside of the "normal," thought it is also eastern orthodox belief that Jesus was born as all of us would have been born had it not been for the fall from Paradise.
We see and accept only a fraction of the possibilities around us, and so we limit our growth and experience. There is a wide universe out there. Be open to it.