single/dual quad set up

ok so i was wondering if most people say that a 650cfm carb is plenty for a 318ci and is alot for a 273ci .then why is it when poking around that looking at a lot of old engine pictures and this and that i have seen alot of 273 with dual quads. what is then gain/effect . what is the point of running 2 smaller carb versus one big one