Nerfs and Buffs

I don't understand almost all of the nerfs and buffs that are happening right now. Champions who don't need to be nerfed are nerfed and champions who don't need to be buffed are buffed. For example viktor isn't that strong right now and he is getting nerfed and xin has greatly benefited from the season 6 items changes so the buff is unneccisary . Champions who have been really strong for a while haven't seen any nerfes besides poppy and champions who are really weak/need reworking haven't seen any love.