It matters because the tradition of the church indicates that She is important. Also, as the Mater Dei, and therefore Queen of Heaven (Revelation) she has a better relationship with Christ, and so is a great person to have on your side, because (based on ancient Jewish custom) the Queen was the King’s mother, and the one person the King practically could not refuse, if someone wanted to appeal to the King, often they would go through the “Queen Mother” who would ask the King on their behalf, this is shown when king Solomon, right after his coronation, when his mother first comes before him as King, she makes to prostrate herself (as all others did), yet he stops her, prostrates himself to her, and has a throne made for her, then when his brother needs to seek his mercy, his brother goes through the Queen, rather than straight to King Solomon).
In answer to the second question, the actual dogma is vague, simply stating that she was taken, body and soul into heaven. In the western church, it is common to believe that when her earthly life came to an end, she was whisked up to heaven before she died, however, in the east, there is a long-standing tradition (and documents from the early church to back it up), that Mary died (possibly voluntarily, or by martyrdom) and then after being dead for a while (the tradition specifies 3 days) she rose, and was taken body and soul into heaven. The actual tradition has a bit more to it, but I think I would have to surpass character limits to explain, either way however (east or west) is possible.