Golf Green

Why are there extra holes of golf greens?
I looked at the British Open this weekend. I can not help noticing that in back or side edge of the greens additional holes. I wonder what the purpose of these extra holes? I know change location the flag of each round of the tournament. Is there extra holes used in the amendments or what? Only a professional golfer is not know. If you Please, no jokes about the practice green, and yes I am a golfer. Thank you. LJK.
What may confuse the holes are the covers that hide the system Watering green. When you see one of these covers is a head recessed sprinkler.











