User blog comment:Hidden Sniper/Mechanics that don't make sense to me - Part 1 Lifesteal/@comment-5907289-20130716102559

Lifesteal and Doran's give ADC a chance to stay in lane. Varus Q, Caitlyn Q, Karma Q, a Draven Q or two, ... they can easily chunk an ADC in lane. Imagine having no sustain, you have to back each time you arrive in lane since you get chunked right away. Having lifesteal/Doran's helps you farming. And you can only buy/carry so many potions.

This poke/chunk creates a window for the side who has chunked: they can zone the enemy from last hits/exp, or go in for the kill should he get too close. The enemy ADC hides at his turret and waits for minions to autoattack to regain HP.

Late game, lifesteal helps you to not get oneshot in teamfights. For you to be in range to autoattack, means you are in range of Ahri, Olaf, Nasus or any other champion really. Of course, you provide high damage, but not in one burst that prevents them from harming you. Without lifesteal, you'd have the HP to take out one target before retreating. With lifesteal, ADCs without massive range/infinite dashes have the chance to do something in a teamfight.