There's a curious tension of sorts, in the Church's relationship to the Apostles, between the importance of the Apostles and what you might call their irrelevance.
Their importance is clear and absolute. One of the four marks of the Church is that she is
apostolic because she is founded on the apostles, in three ways: - she was and remains built on "the foundation of the Apostles," the witnesses chosen and sent on mission by Christ himself; - with the help of the Spirit dwelling in her, the Church keeps and hands on the teaching, the "good deposit," the salutary words she has heard from the apostles; - she continues to be taught, sanctified, and guided by the apostles until Christ's return, through their successors in pastoral office: the college of bishops, "assisted by priests, in union with the successor of Peter, the Church's supreme pastor"
If the Church is not built on the foundation of the Apostles, then she's just blowing smoke.
At the same time, though we know almost nothing about the Twelve as individual men -- who they were, what they thought, where they went, how they died -- we get along just fine not knowing. True, there are stories and legends about all of them that are generally accepted as true, by the pious if not by the historians. But in the end, these stories and legends are beside the point, which is that through the Apostles the Church has received... I was going to write "the Gospel and the Sacraments," but it's shorter and truer to simply say "everything she has and is."
Everything the Church has and is, everything she says, depends on these twelve men -- not as abstract symbols of the tribes of Israel, but as particular, specific individuals who lived particular, specific lives. That they lived particular, specific lives is absolutely critical, if Christianity is to be regarded as more than a set of made-up stories, but the particulars and specifics of their lives have been largely wiped away from the Church's perspective, making the Apostles, as the old image has it, near-perfectly transparent windows onto Christ.