Why are multiple choice questions used in public opinion polls

I would mostly say they would ask something like. "what do you think of this such of a topic"

Lets say Bobby and Fred want to see how many kids in their school prefer Snickers over Twix or the other way around. They want to see who it on which side or who prefers one over the other. They DO NOT want someone coming up and saying they they like them both. They want to see which one they like just a little bit more. What I just said can apply with public opinion polls.