Because he teaches us the outer extent of our emotions. In Die Walkuere alone he has provided the music most likely to drive people to war, plus the pain of a parent punishing a beloved child. Is there a greater moment in opera than Wotan's "Leb' wohl?" Because he teaches us the depth of our own passion. No one expresses the large emotions so clearly as Richard Wagner.
For the full effect it takes a Wotan who is still going strong all the way to the end. This is where the big voices are truly necessary. This was provided for us in todays Met broadcast.