Everyone knows Barcelona has a dominant midfield and arguably the greatest scoring duo in the world with Messi and Villa. Yet, Barcelona's dominant offense is the reason why their defense is is so essential.
Everyone assumes the midfield will control the game, and at some point, Villa or Messi will break open the scoring. Nine times out of 10, Barcelona's offense will be strong, and they will easily outscore their opponent. But even Barcelona isn't perfect and has its bad nights, and they will need to rely on their defense to get them through some of the tough Champions League opponents.
In the past, Puyol was the main fixture of Barcelona's defense. No doubt, he is still valuable and can contribute. But Pique is now the main force in the center, and the success of shutting down teams rests more and more with the Barcelona native.
Barcelona will score goals, but their offense won't be able to completely carry the team to a Champions League title. They will need a solid performance from their defense, and it is Pique that will be the center of that defense, which is why he is essential to Barcelona's ultimate success in these final rounds.