Pro rata is used where the exchange wants to encourage deep quoting rather than speed, typically in short-term interest rate products where thousands of lots rest at every tick. Because size determines fills, market makers post far more than they intend to trade and rely on statistical fill rates.
The consequence for anyone else is that displayed depth overstates real liquidity. A screen showing 20,000 lots bid may thin dramatically the moment an aggressive seller arrives, because the quotes were never meant to be fully hit.
Example: you rest 200 lots in a 10,000-lot queue and a 1,000-lot seller arrives. You receive about 200/10,000 x 1,000 = 20 lots, regardless of whether you were first or last to arrive.
Related: matching-algorithm, sofr-futures, order-book, liquidity, level-2