One of the advantages of playing in a suit contract rather then in No Trump, is that the Declarer has more control. By the use of trumps he can prevent the opponents from running their long suit. Trumps are natural stoppers. Playing in a suit contract also enables the Declarer to get rid of losers by trumping them. And for scoring purposes (especially in a match point pair game), making four of a Major vulnerable is 620, while making 3 No Trump is 600.  
advantage of playing in a suit contract, and one that is often overlooked, is that the trump could be used as Entries. This can be done by using 
high trumps as direct Entries; or by ruffing a card in another suit. Let us examine the examples on the right where the lead is in Declarer's hand and Spades are trump.
In example 1 the high trump is used as a direct Entry to Dummy. The Heart losers can be dumped on the winning Clubs. But when and how to get to Dummy? The only Entry appears to be the A S. But do not be in a rush. For added safety play the K S then the Q S. Then  unblock the Clubs by cashing the A C and the K C. Now get to Dummy by playing a Spade to the Ace which does two things: pulls the last trump and provides an Entry to the good Clubs. 

The highest trump in Dummy in example 2 is the 8 S which unfortunately is not high enough to provide a direct Entry. But perhaps there is another way to get to Dummy and make use of those good looking Clubs. First pull trump by playing the Ace, King and Queen of Spades. (Assume a 3/1 Spade split.) That leaves the J S and 9 S in the hand and the 8 S on the table. Then unblock by playing the A  C which leaves three winning Clubs in Dummy. But how to get there? Answer: with the 8 S. Cash the A H and then commit one of the great Bridge crimes: trump your own winning trick. Yes, there are times, and this is on of them, when the Entry is mor important than winning the trick. So play the K H and trump it with Dummy's 8 S! On Dummy's three good Clubs throw away the hand's three losing Diamonds. Bravo!


          S A  5 3 
          H7  4 
          u  6 
          CQ J 10 9 


         SK Q 9 7 5 
         H9  3 
         u  A 
         CA  K 

        S8 4 3 2 
  u 7 6 
        C  K Q J 2 

        S  A  K Q J
        H A  K 
        u 4 3 2 
        C A