Source: Mr. Bridge
Lead up to the closed hand
When you, declarer, lead a card towards the dummy, both defenders can see the card you have played and dummy’s holding. Usually it is fairly easy for them to work out who should win the trick. But when you lead a low card from a singleton or doubleton in the dummy, it is a different matter.
This is a typical defensive problem:
Suppose your opponents reach 6
after opening a strong no-trump. Partner leads o trump. Declarer wins the lead in the dummy and leads a diamond. What do you do?
On the first deal, if you rise with the ace, West’s high diamonds are ruffed and the 
Q is established for a club discard.
On the second deal, you need to rise with your ace, or declarer will play his king and all you will take is a heart trick. If you win your A there is also a defensive heart trick for you to make.
Do you see my point? On the first deal the advantages of playing a diamond from dummy rather than your hand may not have been obvious, but it gives you an extra chance and makes it difficult for your right-hand opponent to do the right thing.