Chances are, it isn’t. The early Catholic Church did a lot of this kind of thing, where they would claim to have a piece of the cross, or a bone of St Peter in a church. It was just to drive tourism into their churches. If you took all the claimed pieces of the cross and assembled them, it would make far more than one cross.
The way it was explained to me, praying to a Saint to speak to God on your behalf is like asking a friend to pray for you. You could just pray to God yourself, but for some reason, having more people pray for you is better.