Liquidity during trading hours is important because an open market with little liquidity makes it difficult to to sell and buy assets in significant quantities.

Liquidity during trading hours is important because an open market with little liquidity makes it difficult to to sell and buy assets in significant quantities. Even reasonably sized transaction start having massive effects on price. Short-term price stability gets lost and what you ultimately have is a significant increase in transactions costs. Ultimately, liquidity during trading hours is a different question from what hours the exchange is open for trading in the first place.