Tozza Mazza wrote:What do these holes actually do?
as it says in your highlighted text, at this positions the wear of the plank/skidblock is measured.
If you read the complete technical rules, you will find that there is a limit for the max. permitted wear of the skidblock during a race.
Some years ago Schumacher was excluded from the race in Spa (IIRC he won) because his plank/skidblock was worn out more then permitted by the rules.