In an improvised game of cricket - in the park, the garden or in the street or playground - there is usually just one wicket, and only one batsman. At the bowler's end there is often just a single stump, or even a pullover or something to indicate the point from which the bowler bowls.
When the batsman runs, he (or she) first approaches the bowler's end. To be 'run out' a fielder needs to break the wicket toward which the batsman is running. So if he only has time to take one run he needs to give some indication that he is not going for a second run. So he has to say something to indicate that his run is completed.
When I was a kid, well over half a century ago, we used to shout 'WICKETS'. But I just noticed last night, in Manchester, playing with my grandson and his mates that they shout 'IN'.
Does anyone else, from one of the great cricket-loving nations, have a view and any particular experience to report in this matter?