LoL... that line reminds me of something from FF Advent Children... ^_^
But anyways, all the reasons revolve around this statement:
The ppl that join and make up the kingdom they will help decide how the Kingdom of ZU Knights works... also the Kingdom will revolve around helping ppl with their problems and talking amongst' the Kingdom's members about any rumors that are goin' around ZU and anticipation of the outcomes of any events that may pop up...
In addition, the Kingdom of ZU Knights stands for the honor and pride of ZU, we will protect ZU and it's members during battles (such as the one between ZI and ZU right now

)... that's optional though

... and overall to have fun! meet new people and make new friends
