Why do heavyweights in boxing need to weigh in?

I wish they wouldn't. Most of them aren't in shape for fighters.

They really don't. It's just a formality. A heavyweight can't be overweight and there is no minimum weight they have to weigh. Bob Fitzsimmons weighed 167 pounds as the world heavyweight champ and Tommy Burns weighed 170 pounds as the world heavyweight champ.

As with all weigh-ins, we want to see their c*cks. The weigh-in stage, aka the snake pit

As with all weigh-ins, we want to see their c*cks.

Just formalities. Showing off for their fans. Although it backfires with a lot of HWs these days because they look like the Michelin Man.