Lots of boring judicial history there but the main gist is that we (the U.S.) created RTOs (like PJM) to ensure that every bit of electricity in a region was being managed by one overall regulator to maximize synergies. Every provider is required to bid (basically) 100% of their capacity on the day-ahead market, every day. If we didn’t force energy bids like this, providers could e.g. withhold energy until peak demand times to maximize profits, essentially giving an F U to people at non peak times and leaving them susceptible to blackouts. The bidding system is just the method that FERC employs to find a fair price to pay providers despite there being so many - but in reality, it’s just the chosen strategy to ensure all electricity being produced enters the market.