Well, I don't know in ou, as that should be impossible(it's banned in ou) but as for ag there is a reason. Basically, water spout is a 150 power move if it's at full health. Primal kyogre gets a STAB boost, a heavy rain boost(100%) and it's a 150 power move, so huge power there. It also has an amazing 180 base special attack, which is very high. It also has thunder, a 110 power electric type move, that doesn't miss in primal kyogres constant rain, and ice beam, a nice coverage move. It has decent speed, and can use this to somewhat sweep a team with its, like I said, stab rain boosted 150 power move. In all, it's insane sp atk and the rain boost and the STAB and the fact that it's a 150 power move makes it pretty powerful. (I also want to add that most also carry orgins pulse/surf(one of the two) a 110 power stab move also boosted by rain(orgin pulse) or a 90 power STAB rain boosted move(surf), which it can use if it gets damaged from attacks and water spout is weaker)
Hope I helped!