"Did you push towers?" "Did you secure neutral objectives?"
Killing enemies prevents them from securing objectives themselves, and makes it easier for everyone else on your team to secure these objectives without getting ganked.
"Did you ward?"
The purpose of warding is to know where the enemy is. Killing the enemy lets you know where the enemy is.
"Do you have good CS count?"
If somebody goes 5/0/5 and they do it on 0 CS, that's better than if they do the same thing on 100 CS. CS is a resource and if you can be a useful member of your team without consuming that resource, that's a good thing, because it means that somebody else can farm in your lane while you get a'killin'.
KDA isn't the only way you can contribute to a game, but it is flat out wrong to think that rendering enemies incapable of pushing, farming or securing objectives is "almost irrelevant".