In what sense am I a witness of the death and resurrection of Jesus? And why does it matter? First question first.
Let me begin subjectively because we are "all about" subjective experience these days. I am a witness to the death and resurrection of Jesus every time that I let whatever is death-dealing in my life be overcome by love and mercy. This is especially true of my own failures. In this sense, literally anyone can be a witness of the death and resurrection of Jesus, even without knowing it. If anyone can receive these graces, then how do I know that they come from the pierced heart of Jesus? Because I am also an objective witness of the death and resurrection of Jesus. I believe that I am an actual objective witness of the death and resurrection of Jesus -- as much as the Apostles were -- through the Mass. Although present under sacramental signs, I believe that at Mass I actually become a witness to the saving events of the life of Jesus. I receive directly from Jesus His own flesh and blood. There is thus no doubt for me where the graces of salvation come from. I believe that the objective reality of the Mass is why people are drawn to the Mass, even without faith or with weak and faltering faith. It is what it is regardless of the response. All the better if I am ready to recognize the Mass for what it is.
It matters because love and mercy are the only way out of the misery of sin and death. Christians propose love and mercy in the flesh of Jesus. Catholics propose the flesh of Jesus in the Mass.