Holes on Bridge

In the rule 2.5.5 it says: “The top surface of each bridge includes an array of 15 small holes.”

Does anyone know what purpose these holes serve? Are they simply there to change the mass or do they have a different purpose?

I believe they are to hold the extra balls that will be stored on the ramp at the beginning of the match. Think golf tee

Sounds like a resonable explanation. Thanks!