Open outcry existed to make the pit fair: bids and offers had to be announced aloud to the whole crowd, not whispered to a friend. Hand signals conveyed price and size when the noise made speech useless, palms out to sell and in to buy.
Its regulatory descendants survive in modern rules against prearranged trading and in the requirement that block-trades and efrps meet strict conditions to trade off-book.
Example: a broker with 200 lots to buy shouted the bid to the whole pit; anyone could hit it. The electronic equivalent is a resting bid on globex visible to every participant.
Related: pit-trading, locals, block-trade, efrp, globex