"Bouncing" refers to sending a card from the field to the hand. Most meta-decks use the cards in their graveyard extensively (Grapha, Dragon's Mirror, BLS, Wind-Up Rat, etc.), so keeping cards off the field and out of the graveyard is that much more effective. This also prevents the use of card effects that activate when a card is destroyed or sent to the graveyard (Sangan, Lightpulsar Dragon, Machina Fortress, etc.). An added benefit of this theme is that they can escape the several cards that counter destructive effects that wouldn't counter bouncing effects (Stardust Dragon, Starlight Road, etc.).