Any fervent Catholic will tell you: devotion to the Virgin Mary is the key mark of a true Catholic. The reason is quite simple: below Christ Himself, Mary is the highest spiritual authority that we as Christians can turn to for guidance and support.
Now the Protestant question: Why can't we just go straight to Jesus? It seems to work just fine.
So if the question is one of proximity to our Lord Jesus, then of course, not a single creature in the whole universe can boast of such closeness as His mother. Indeed, she is too close for comfort for many non-Catholics. Chances are that as Catholics, many if not most Protestants we meet will consider it idolatrous and heretical to summon Mary's name - many of these same Protestants, though, probably don't agree with my former pastor's assessment that all of Christ's original followers in the New Testament are indeed closer to Him than we are. "Oh, they're just dead saints" is probably how they see them.
But as Catholics, it is a matter of faith for us that the Virgin Mary was assumed into heaven upon her death and has thus enjoyed a beatific view of the Holy Trinity for the past two millennia. As such, she is in a uniquely good position to hear the supplications of those who seek the mercy of her Divine Son. While anyone can call on Christ directly (and even as Catholics, we should), a refusal to acknowledge Mary's intercession will leave the following gaps in our spiritual life:
1) An incomplete understanding of the depth of God's purity2) An incomplete understanding of the depth of God's mercy
3) An incomplete understanding of God as a Trinity
The aim of the Christian life is to become sinless, to the extent that our Holy Mother is sinless and thus worthy to enjoy the vision of God's glory. May we be filled with joyful hope that we will be in the company of such a beautiful woman in the age to come.