I don't understand why people even build 
cuz it gives less attack spd than every other attack speed crit item and ik its good for turrets but in duels wouldn't it be better to just take statikk, like it gives u more attack spd and ms and anyway if you have more attack spd you are going to take down turrets faster so why is rapid fire even good?
More reach means you can threaten somebody before they can threaten you. An extreme example of this is Tryndamere vs any Ranged ADC. Tryndamere will stat check the hell out of the vast majority of Marksman, but it doesn't matter since he can never reach them.
Couple this with some ADCs that can do a lot in just one hit (Ashe slow, Draven or Cait empowered autos, Enabling Kalista's leap) and you have a selective but valuable niche for this item.
On that note, Statikk and Rapid Fire Cannon are both inferior for dueling compared to Phantom Dancer, because dueling is PD's niche, not Statikk nor technically Rapidfire.
You get the first hit. And on some ADCs like a late game Jhin, all you need is to get the first hit, and it'll be the last hit.
