Now eventing is quite the test of a horse? Why was it started anyways?
Is is for the Horse? or for the Rider to prove something? Not knocking it by anymeans if it sounds that way..
I am just wondering as I do from time to time.. what kicks off these "denominations" in equine sport.

Eventing, aka, combined training, traces its roots to cavalry horse trials, much like dressage. In eventing, the horse and rider compete in three phases--dressage, stadium jumping and cross country jumping. At the upper levels, the cross country course is extraordinary and requires the boldest and bravest hearts (of both horse and rider!) I see a real eventer as the ultimate athlete that can perform upper level dressage and then faces cross country obstacles that drop feet on the other side into water. It amazes me that a horse will jump something when he can't see what's on the other side! I was just watching an old clip of an event in which one rider was returning to competing after fracturing vertebrae in her neck in a previous eventing accident three months prior! Crazy or gutsy? You got me. I've never had a horse who was talented enough in all three phases to compete in eventing. For a great show of talent in both dressage and jumping, watch the O'Connors...the husband and wife team of David and Karen O'Connor, US Olympic team members and medal winners. They are very impressive in all phases of eventing...as are most who even make it to that level.

The thing that confuses me is that the eventing dressage tests are different than the regular dressage tests. What's the sense in that?

what is the injury rate in a sport like this... it is almost a little extreme on the old knees? I mean I have seen some down hill jumps that look like they could easily cripple a bad landing... easily.

Yes everybody is right. Eventing started as a way of choosing the best horses for the calvary. Dressage a test of trainability,x-country to test stamina and boldness and stadium jumping to test accuracy and soundness as it is always run last.
I ride both Dressage and eventing and don't understand the multiple test either. I think its an Ego thing.
Injury rate,who's knees ? The horses are extremely well managed and have a competitive carreer into their late teens. The riders- we won't go there.