Why the holes in the fifth valve plate?

The fifth valve was designed to be customizable. Not only can the tension be changed with more or fewer resistance bands, the position of the thumb rod can be rotated to suit the player’s hand. The thumb ring can likewise be adjusted. The down side, of course, is that the thumb ring is not a handle for picking up the tuba. Some players, when they have the position they prefer, may chose to have the thumb ring soldered in place.