When you take the wheel as declarer, you have but one mission: to make your contract. All other considerations, namely overtricks, are secondary. Why it is necessary to make a statement which seems so obvious? History proves that it’s not so obvious to some. Look at these two hands:
You are in 3NT, and West leads the 
Q. You win and stop to count your tricks. You have the top two spades, the 
A, the 
A, and the top three diamonds. That’s seven tricks — two short of your goal. But what about that lovely diamond suit in dummy? It’s really worth more tricks than just the top ones, right?
If one opponent has three diamonds and the other opponent two, the suit will “run,” as bridge players say — it will be good for six tricks. So six diamonds, two spades, and one each in clubs and hearts make 10 tricks. An overtrick!
Not so fast.
What happens if you play the 
A, then the K and one of your opponents shows out. That means someone has four diamonds. The suit isn’t running after all, but your problems are bigger than that. If you cash the Q and play another one, making the last two diamonds in dummy winners, it will all be for nothing because you won’t be able to get to them.
Plan for the Worst
That dummy, with two good diamonds sitting there, is as dead as a doornail. You’ll never be in dummy again. Not only that, but you have no way to develop extra tricks in the other suits, so you are headed for a minus score on a deal that should produce nine tricks most of the time.
Do you see what went wrong? You failed to plan for a potential bad break. It’s true that the odds favor a 3-2 split in diamonds, but you should assume the worst and plan accordingly. You may give up an overtrick now and then, but that’s a small price to pay for assuming that, whenever the contract can be made, you will make it.
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