If I watch say Boston College playing Virginia, I don't care who wins, I try and look at the MLB or another player to see how they react. I'm still puzzled by how they read the offensive alignments, often, to me, the offensive presnap looks the same as nearly every other play.
It's clear the D reacts to a player who motions. I infer the QB can read that reaction and figure what sort of defense they are in, etc.