Just because deer are colour blind does not make camo useless. Breaking up your pattern is helpful for most hunting situations. That can be done with things other than camo, but camo will never hurt your hunting. That is why plaid works well, it breaks up your body to make it blend better. Most quality outdoor hunting clothes are camo now anyway, so it gets harder to find good warm quiet clothing that isn't camo.
There is little downside to it, other than cost, but that is minor anymore since most hunting stuff is made in camo.
Certainly isn't necessary to kill animals. But again will never hurt your hunting and blending into your surroundings is a definite step up in your hunting game. There will always be exceptions, but overall you will see more game the better you can blend in. Camo can be a big part of that. Along with scent control and limiting movement.