here is no need to spend money on advertising, because there is perfect knowledge and firms can sell all they can produce. In addition, selling unbranded goods makes it hard to construct an effective advertising campaign.
There is maximum possible:
Consumer surplus
Economic welfare
There is maximum allocative and productive efficiency:
Equilibrium will occur where P = MC, hence allocative efficiency.
In the long run equilibrium will occur at output where MC = ATC, which is productive efficiency.
There is also maximum choice for consumers.