When you have a long trump suit, it may be a major source of transportation, particularly if Dummy is rather weak. Keeping one or two small cards in your trump suit may be imperative.
You hold: ♠953 ♥J ♦KJ9873 ♣AQ10. RHO opens 1♠ and you overcall 2♦. LHO passes and partner raises you to 3♦.
Lead is the ♠J and Dummy produces: ♠KQ ♥A954 ♦654 ♣8652.
You play a spade. RHO takes ♠JA and returns the ♠7 as LHO follows with the ♠6. You play the ♦4 and RHO takes Ace and leads the ♠10. Your most important play was to play the ♦9, ♦8, or the ♦7 under RHO's the ♦A! You need to keep your precious the ♦3 as a possible entry to Dummy's ♦6.
Since RHO has just led the ♠10, you expect that LHO is ruffing, as she does indeed with the ♦10 and returns the ♥6 (as requested by her partner's the ♠10—suit preference when giving partner a ruff). You rise the ♥A and play a low club to the ♣Q, expecting RHO to have the ♣K. Alas, LHO takes the ♣K and returns a heart to partner's ♣Q. (RHO dropped the ♥K under the Ace.) You duly ruff that heart with the ♦9, ♦8, or the ♦7, retaining your precious the ♦3. You play the ♦K, dropping the now bare ♦Q on your left.
Your the ♦3 goes to the ♦6 on Dummy and you play a club to your ♣10 to rack in nine tricks and a plus score.
Unless you plan from the beginning to cherish that precious the ♦3, you will go down one, losing a second club trick in the end game.