Usually when playing a notrump contract, you aim to set up a long suit for more tricks. Setting up the suit requires that you have an entry to it once it is set up. And you often must give up a trick or two before your long suit gets set up. Here is a recent example:
You are West in 3NT, and the opening lead is the spade four (4th highest from opponents’ longest suit). Counting your tricks, it looks like you have three sure spade tricks, two sure diamond tricks and one sure club trick for a total of six tricks. Once you drive out the king of clubs, the club suit looks like it may be a good source of more tricks.
But be careful of your entries to the clubs. The king of spades is a sure entry, so don’t squander it early. Win the opening lead in hand (you did play low from dummy didn’t you?). Start the clubs by leading the ace and then the queen. The only time you will have difficulty in setting up clubs for four tricks is the unlikely holding of all five missing clubs by one defender. As the cards lie you will easily take three spade tricks, two diamond tricks, and four club tricks for nine total.