There are any number of times and situations where an official needs, or may need, to mark a spot and two ways to do so. First would be to stand at the particular spot you need to mark, and when you cannot "stay" at the spot, you need to mark or may need to subsequently reference, to drop a bean bag at the spot to mark it as you continue officiating.
At the professional level the custom is to mark certain out of bounds spots with their hat, rather than a bean bag.
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