The guard is a shield for jiu jitsu players and can be used as an equalizer against a more powerful opponent.
The Half Guard is the link between the full guard and many other unfavorable positions.
The Cross-side position is one of the more dominant positions a top player can find himself in.
The Mount is one of the most dominant postitions for the top player.
Back control is arguably the most dominate position one can attain in a fight.