Suppose you're playing Annie. Suppose you dive the enemy team with your stun proc and ult up, and R-W them and are instapopped. You kill no one, but the entire enemy team is chunked to numbers like 25%-50% remaining HP, and then your team finishes all of them off with no casualties. That's one death and zero kills, yet so much more valuable than if you were trying to maximize your k/d ratio and chose not to suicide engage.
Take another instance - suppose you're playing TF. You use your ult to teleport to a lane with no enemies in it while they're contesting dragon and manage to take down a tower and an inhibitor, but end up dying in the process. Your pressure forced them to withdraw from drag and cost them an inhibitor, yet you killed no one and died yourself. Still worth.
Suppose you're Nunu and the enemy team is taking baron, but your team is in no position to contest. At the last second you flash in and Q-Smite baron to secure the buff for your other 4 teammates and prevent the enemy team from acquiring it. You die in the process and take no one with you, but now the enemy team is down 1500 gold and doesn't have a baron buff.
This is not CoD. Don't focus solely on k/d ratios to determine your worth as a player.