ADC First Item Rapid Fire Cannon?

I've been playing a lot of games and watching streams lately. As an ADC main I've gotten used to the first items on pretty much all the basic ADC's (ex. Lucian: ER Kalista: BORK). Any way champions like Tristana, who i would start IE on, and Vayne, who I would start BORK on, I've now seen multiple times (specifically on imaqtpie's stream but in game too) being started with Rapid Fire Cannon. Can someone explain to me why this is a preferable starting item for some champs now. Thanks in advance.

Edit: Spelling Error, oops