Strategy in the game of Hearts is mainly focused on the Queen of Spades because of how much it is worth. If a player is "Shooting the Moon" for example, another player might consider taking the Queen of Spades (13 points) rather than allowing it to go to the other player and risking 26 points instead if that player ends up successful in their "moon" attempt.
When passing, most players will retain their low spades as they may be required to help protect a player holding the Queen of Spades. This gives the player flexibility in unloading the Queen at the most opportune time.
It is generally considered a mistake to pass spades unless the Queen is passed in combination with the king and/or the ace. Some will pass off diamonds or clubs in hopes of having an opportunity to rid one's hand of a penalty card, but this strategy is not always successful depending on what cards are passed prior to the start of the game.
Passing and game strategies vary depending on player experience and skill, and of course on what cards are originally dealt and then passed. Much of what strategy will work has to do with so many other game variables, making it impossible to predict but also adding to the excitement of the game.
