Let's talk about Runaan's again

Let me preface this by saying I love this item. In fact, I'm upset about losing the reload passive because I loved throwing both of these items on TF and laughing because I have DPS AND burst. And I always used to argue that Runaan's belongs on way more ADCs, but... why are people building this thing first item? I get it if you think it's core to your character (looking at you, ), but first item?

Does really provide a more meaningful DPS increase in the LANING PHASE than, say, literally anything else you can buy for 2600g? I get that it sort of saves you the price of boots early, but I've been in several games now as a "support" and my ADCs are almost RUSHING this item. Does it help you farm? Does it make you win trades (outside of bullshit crit streaks)?

I'm not saying stop building this item. I'm saying that if you come back to lane with anything other than an AD or Life Steal item, I die a little on the inside. You're making my job harder.

Help me understand why this item is so important as to buy first. If it isn't important, please stop buying it first. I've played games where my ADC had this item before 15 minutes, and I can't fathom why they bought this instead of, say