To discover the distribution of the unseen hands either as declarer or defender.
You and your partner bid all the way to 7♠ (or you may be playing matchpoint pairs and be keen to make an overtrick in 6♠).
West leads a trump and East follows. You play a second round of trumps and all follow.
To maximise your chance of working out who has the ♣Q you want to find out how many clubs each defender has – whoever has more clubs is the favourite to hold the queen.
You cash the red suit tops, ending in dummy and ruff a heart. You then crossruff the red suits, noting what happens.
Suppose all follow to four rounds of hearts but East shows out on the fourth round of diamonds. In this case, West’s shape is 2-4-4-3 and East’s is 2-4-3-4, so you play East for the ♣Q.