We may look at a suit such as T432 and think we have no tricks there so we can safely discard the 2. However, that might let the contract through! How do we know?
At the table, we often come across hands where the Declarer has all but one trick that she needs for her contract. There are many ways to look for that extra trick, some more technical and others, less so, however, the best way to go about it is to maintain a basic discipline.