Imagine we sell groceries. If I have real-time access to your prices, and you to mine, then I have no incentive to try to outbid you to my customers since you will see my cheap potato sale sooner than my customers will, and announce your own equivalent sale. Knowing you have access to my prices, I can also increase them safe in the knowledge that you will follow me very soon, since that means we both win. That is how sharing prices with no friction leads to less competition.