This is arguably the be-all / end-all of PvE tanking. While other combinations may be better in certain situations, the Knight / Warrior has no discernible weaknesses no matter who they're up against. It also holds a special place in certain PvP objectives due to its sheer survivability, making it a combination that is highly desired for groups.
Strength and Stamina are the greatest attributes for this, and being able to dump those stats into gear gives it an overwhelming advantage over other combos that are more "hybrid"; for example, Knight / Mage needing Intelligence, or Knight / Scout wanting Dexterity. Holy Strike and Punishment deal with bosses, Whirlwind and Whirlwind Shield make it effective at handling groups, while Shield of Valor and Shield of Discipline help with survival.
While tanking should be practiced, Knight / Warrior is a very forgiving combo for PvE scenarios given its sheer power and endurance. Using Holy Strike->Punishment for bosses and the Whirlwind skills for adds and groups will let you maintain high aggro and keep your teammates safe from enemies' attention.