The late nobel prize-winning economist george stigler once wrote, "the most common and most important criticism of perfect [is] that it is unrealistic."
source: george stigler, "perfect competition, historically contemplated," journal of political
economy, vol. 55, no. 1, (february 1957), pp. 1-17.

despite the fact that few firms sell identical products in markets where there are no barriers to entry, economists believe that the model of perfect competition is important because

The perfect competition model is based on the assumption that many companies provide the same type of product or service. In this competitive model, companies are not large enough to be able to influence prices, that is, the price is determined by the market through the interaction between demand and supply.

Economic reality is complex and would hardly be portrayed in a real way. That's why economists use models that can clarify concepts and create insights into how the real economy works. That is why the perfect competition model demonstrates what a liberal competitive economy would look like.

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