The marker's job
Under the Rules of Golf (Rule 3.3b), the marker enters your score for each hole on your scorecard and certifies it. Usually it's someone in your group, and you mark theirs.
Your job
You check the hole scores, certify them yourself, and hand the card in. If a hole score is wrong, it's your responsibility, not the marker's.
Why leagues use one
A score two people have put their name to settles most arguments before they start. It's also why a card kept by a marker and signed by the golfer is trusted in TourCard without a second look.