Do you have a double or a triple? I have a triple, and I know it's time to go up to the big ring when I'm getting chain rub on my middle ring. If I want to keep pedaling and go faster, I need a gear that's harder to pedal and I can't get one in the middle ring because it's rubbing. So my choice is to coast and not pedal, or switch to the big ring so I can keep pedaling. Does that make sense? So if I was on a flat and wanted to go faster, I might choose the big ring. Or if I was on a downhill and wanted to pedal instead of coasting, I would really have to go up to the big ring.